I traced the CorrelationID and the actual error is:

List does not exist.  The page you selected contains a list that does not exist.  It may have been deleted by another user.

The reason I am importing the workflow into a new site, is that I want to change it so that it references list locations on another site.  

Usually this shouldn't be a problem. You can import a workflow in a site where referenced lists and fields do not exist. However the actions referencing such lists and field will have an invalid configuration and you will not be able to publish the workflow.

Unfortunately, it is a problem. I am trying to create a development site so I don't have to make changes to production until I know that it works.  I used the production list to create a template.  I exported the list template to the new site collection and created a list.  I then exported the production workflow.  When I try to import this production workflow, I receive the ambiguous "unexpected error" with a correlation id.  When I trace that ID, it says that the List does not exist, but does not say what list it is expecting.

Additional info.  Here's the correlation information from the latest attempt:

Area : SharePoint Foundation
Category : Logging Correlation Data
Level : Medium
EventID : xmnv
Message : Name=Request (POST:http://spdev:80/apps/salesorder/_layouts/15/NintexWorkflow/Im
portWorkflow.aspx?Category=List&ListId=e3fa70c5-37d7-412f-a507-03a8041871ea&Cont
entTypeId=&IsDlg=1)

Area : SharePoint Foundation
Category : Authentication Authorization
Level : Medium
EventID : agb9s
Message : Non-OAuth request. IsAuthenticated=True, UserIdentityName=0#.w|reipetersend,
ClaimsCount=47

Area : SharePoint Foundation
Category : Logging Correlation Data
Level : Medium
EventID : xmnv
Message : Site=/apps/salesorder

Area : SharePoint Foundation
Category : Database
Level : High
EventID : ahjqp
Message : Forced due to logging gap, cached @ 10/13/2017 09:59:52.12, Original Level:
Verbose] SQL connection time: 0.137517477779997 for Data Source=SPDEV;Initial
Catalog=WSS_Content_REIINFO_BAK;Integrated Security=True;Persist Security
Info=False;Enlist=False;Pooling=True;Min Pool Size=0;Max Pool
Size=100;Asynchronous Processing=False;Connection
Reset=True;MultipleActiveResultSets=False;Replication=False;Connect
Timeout=15;Encrypt=False;TrustServerCertificate=False;Load Balance
Timeout=0;Packet Size=8000;Type System Version=Latest;Application Name=".Net
SqlClient Data Provider";User Instance=False;Context
Connection=False;Transaction Binding="Implicit Unbind";ApplicationIntent=ReadWri
te;MultiSubnetFailover=False;TransparentNetworkIPResolution=True;ConnectRetryCou
nt=1;ConnectRetryInterval=10;Colum...

Area : SharePoint Foundation
Category : General
Level : High
EventID : g3qe
Message : Forced due to logging gap, Original Level: Verbose] GetServerRelativeUrlFromUrl
(http://spdev/apps/salesorder/_layouts/15/NintexWorkflow/ImportWorkflow.aspx,Tru
e,False)

Area : SharePoint Foundation
Category : Database
Level : High
EventID : ahjqp
Message : Forced due to logging gap, cached @ 10/13/2017 09:59:52.17, Original Level:
Verbose] SQL connection time: 0.156234940474278 for Data Source=SPDEV;Initial
Catalog=WSS_Content_REIINFO_BAK;Integrated Security=True;Persist Security
Info=False;Enlist=False;Pooling=True;Min Pool Size=0;Max Pool
Size=100;Asynchronous Processing=False;Connection
Reset=True;MultipleActiveResultSets=False;Replication=False;Connect
Timeout=15;Encrypt=False;TrustServerCertificate=False;Load Balance
Timeout=0;Packet Size=8000;Type System Version=Latest;Application Name=".Net
SqlClient Data Provider";User Instance=False;Context
Connection=False;Transaction Binding="Implicit Unbind";ApplicationIntent=ReadWri
te;MultiSubnetFailover=False;TransparentNetworkIPResolution=True;ConnectRetryCou
nt=1;ConnectRetryInterval=10;Colum...

Area : SharePoint Foundation
Category : Web Controls
Level : High
EventID : 90i3
Message : Forced due to logging gap, Original Level: Verbose]
PersonalActions::CreateChildControls() - Was called.

Area : SharePoint Foundation
Category : General
Level : High
EventID : g3qj
Message : Forced due to logging gap, cached @ 10/13/2017 09:59:52.25, Original Level:
Verbose] url is in site

Area : SharePoint Foundation
Category : Database
Level : High
EventID : 8acb
Message : Forced due to logging gap, Original Level: VerboseEx] Reverting to process
identity

Area : SharePoint Foundation
Category : General
Level : High
EventID : 8kh7
Message : List does not exist. The page you selected contains a list that does not
exist. It may have been deleted by another user.

Area : SharePoint Foundation
Category : General
Level : High
EventID : aix9j
Message : SPRequest.GetListsWithCallback:
UserPrincipalName=i:0).w|s-1-5-21-2307382639-3124176078-3801479691-1884,
AppPrincipalName= ,bstrUrl=http://spdev/apps/salesorder/_layouts/15/NintexWorkfl
ow/ImportWorkflow.aspx?Category=List&ListId=e3fa70c5-37d7-412f-a507-03a8041871ea
&ContentTypeId=&IsDlg=1 ,foreignWebId=00000000-0000-0000-0000-000000000000
,bstrListInternalName={8819CA74-37C7-44E7-B421-68181F31F9F8} ,dwBaseType=-1
,dwBaseTypeAlt=-1 ,dwServerTemplate=-1 ,dwGetListFlags=0
,dwListFilterFlags=4294967295 ,bPrefetchMetaData=True ,bSecurityTrimmed=True
,bGetSecurityData=True ,bPrefetchRelatedFields=False

Area : SharePoint Foundation
Category : General
Level : Medium
EventID : ai1wu
Message : System.Runtime.InteropServices.COMException: List does not exist. The page you
selected contains a list that does not exist. It may have been deleted by
another user., StackTrace: at
Microsoft.SharePoint.SPListCollection.EnsureListsData(Guid webId, String
strListName) at
Microsoft.SharePoint.SPListCollection.EnsureListsData(String strListName)
at Microsoft.SharePoint.SPListCollection.ItemByInternalName(String
strInternalName, Boolean bThrowException) at
Microsoft.SharePoint.SPListCollection.GetListById(Guid uniqueID, Boolean
bThrowException) at Nintex.Workflow.ImportContext.1R4=() at
Nintex.Workflow.ApplicationPages.ImportWorkflow.btnSubmit_Click() at
Nintex.Workflow.ApplicationPages.ImportWorkflow.RaisePostBackEvent(String
eventArgument) at System.Web.UI.P...

Area : SharePoint Foundation
Category : Database
Level : High
EventID : ahjqp
Message : Forced due to logging gap, cached @ 10/13/2017 09:59:52.34, Original Level:
Verbose] SQL connection time: 0.127111127252207 for Data Source=SPDEV;Initial
Catalog=WSS_Content_REIINFO_BAK;Integrated Security=True;Persist Security
Info=False;Enlist=False;Pooling=True;Min Pool Size=0;Max Pool
Size=100;Asynchronous Processing=False;Connection
Reset=True;MultipleActiveResultSets=False;Replication=False;Connect
Timeout=15;Encrypt=False;TrustServerCertificate=False;Load Balance
Timeout=0;Packet Size=8000;Type System Version=Latest;Application Name=".Net
SqlClient Data Provider";User Instance=False;Context
Connection=False;Transaction Binding="Implicit Unbind";ApplicationIntent=ReadWri
te;MultiSubnetFailover=False;TransparentNetworkIPResolution=True;ConnectRetryCou
nt=1;ConnectRetryInterval=10;Colum...

Area : SharePoint Foundation
Category : General
Level : Medium
EventID : 8nca
Message : Application error when access /_layouts/15/NintexWorkflow/ImportWorkflow.aspx,
Error=Data at the root level is invalid. Line 1, position 1. at
System.Xml.XmlTextReaderImpl.Throw(Exception e) at
System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() at
System.Xml.XmlTextReaderImpl.ParseDocumentContent() at
System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean
preserveWhitespace) at System.Xml.XmlDocument.Load(XmlReader reader) at
System.Xml.XmlDocument.LoadXml(String xml) at
Nintex.Workflow.Activities.Adapters.NWQueryListAdapter.OnImport(NWActionConfig
activityConfig, ImportContext importContext) at
Nintex.Workflow.ActivityReference.OnImport(NWActionConfig activityConfig,
ImportContext importContext) at Nintex.Workflow.WorkflowRenderer.DSg=(...

Area : SharePoint Foundation
Category : Runtime
Level : Unexpected
EventID : tkau
Message : System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
at System.Xml.XmlTextReaderImpl.Throw(Exception e) at
System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() at
System.Xml.XmlTextReaderImpl.ParseDocumentContent() at
System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean
preserveWhitespace) at System.Xml.XmlDocument.Load(XmlReader reader) at
System.Xml.XmlDocument.LoadXml(String xml) at
Nintex.Workflow.Activities.Adapters.NWQueryListAdapter.OnImport(NWActionConfig
activityConfig, ImportContext importContext) at
Nintex.Workflow.ActivityReference.OnImport(NWActionConfig activityConfig,
ImportContext importContext) at
Nintex.Workflow.WorkflowRenderer.DSg=(NWActionConfig Dig=, HtmlContainerControl
Dyg=, Guid ECg=, ...

Area : SharePoint Foundation
Category : General
Level : High
EventID : ajlz0
Message : Getting Error Message for Exception System.Web.HttpUnhandledException
(0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown.
---> Nintex.Workflow.NWRenderWorkflowException: Error rendering workflow
segment. ---> System.Xml.XmlException: Data at the root level is invalid. Line
1, position 1. at System.Xml.XmlTextReaderImpl.Throw(Exception e) at
System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() at
System.Xml.XmlTextReaderImpl.ParseDocumentContent() at
System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean
preserveWhitespace) at System.Xml.XmlDocument.Load(XmlReader reader) at
System.Xml.XmlDocument.LoadXml(String xml) at
Nintex.Workflow.Activities.Adapters.NWQueryListAdapter.OnImport(NWActionConfig
activityConfig, Impor...

Area : SharePoint Foundation
Category : General
Level : Monitorable
EventID : aat87
Message :

Area : SharePoint Foundation
Category : Micro Trace
Level : Medium
EventID : uls4
Message : Micro Trace Tags: 0 nasq,10 agb9s,237 aix9j,0 ai1wu,73 8nca,0 tkau,0 ajlz0,1
aat87

Area : SharePoint Foundation
Category : Monitoring
Level : Medium
EventID : b4ly
Message : Leaving Monitored Scope (Request (POST:http://spdev:80/apps/salesorder/_layouts/
15/NintexWorkflow/ImportWorkflow.aspx?Category=List&ListId=e3fa70c5-37d7-412f-a5
07-03a8041871ea&ContentTypeId=&IsDlg=1)). Execution Time=333.976594790679

I am facing form load issue. It was working before in my production enviornment and stopped now.

The error is 

>XmlException: Data at the root level is invalid. Line 1, position 1.] System.Xml.XmlTextReaderImpl.Throw(Exception e) +88 System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() +6367311 System.Xml.XmlTextReaderImpl.ParseDocumentContent() +114 System.Xml.XmlReader.MoveToContent() +98 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderDateTimeValue.Read4_DateTimeValue() +28 InvalidOperationException: There is an error in XML document (1, 1).] System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events) +819 System.Xml.Serialization.XmlSerializer.Deserialize(TextReader textReader) +157 Nintex.Forms.Serializer.DeserializeXml(String data, Type type) +122 Nintex.Forms.NfUtilities.ParseDateTime(String dateTimeString, CalendarType calendarType) +121 Nintex.Forms.FormsHelper.PrepareDateTimeFormControlProperties(Form form, ContextDataCollection data, CalendarType regionalCalendarType) +262 Nintex.Forms.SharePoint.Helper.FormPreprocessorAddContextData(String& serializedForm, Collection`1 variables, PublishingType publishingType, ContextDataCollection data) +87 Nintex.Workflow.Forms.ControlTemplates.ListForm.InitialiseFormFiller(String formXml, FormData data, Boolean hasEnterpriseLicense) +372 Nintex.Workflow.Forms.ControlTemplates.ListForm.InitialiseForm(Boolean hasEnterpriseLicense) +250 Nintex.Workflow.Forms.ControlTemplates.ListForm.OnInit(EventArgs e) +513 System.Web.UI.Control.InitRecursive(Control namingContainer) +433 System.Web.UI.Control.InitRecursive(Control namingContainer) +219 System.Web.UI.Control.AddedControl(Control control, Int32 index) +239 Microsoft.SharePoint.WebPartPages.ListFormWebPart.CreateChildControls() +301 System.Web.UI.Control.EnsureChildControls() +130 Microsoft.SharePoint.WebPartPages.WebPart.get_WebPartMenu() +25 Microsoft.SharePoint.WebPartPages.ListFormWebPart.CreateWebPartMenu() +44 Microsoft.SharePoint.WebPartPages.WebPart.OnWebPartMenuCreate(Object sender, EventArgs s) +50 System.Web.UI.Control.OnLoad(EventArgs e) +86 Microsoft.SharePoint.WebPartPages.ListFormWebPart.OnLoad(EventArgs e) +50 System.Web.UI.Control.LoadRecursive() +129 System.Web.UI.Control.LoadRecursive() +226 System.Web.UI.Control.LoadRecursive() +226 System.Web.UI.Control.LoadRecursive() +226 System.Web.UI.Control.LoadRecursive() +226 System.Web.UI.Control.LoadRecursive() +226 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3712