Symptoms

P and D throwing the error when creating a package:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Runtime.Serialization.SerializationException: Member 'SessionName' was not found.
at SourceCode.Hosting.Client.BaseAPI.BaseAPIConnection.RemoteCall(String TypeName, String MethodName, Objectt] Parameters, Booleann] NullList, MarshalMessageType CallType)
at SourceCode.Hosting.Client.BaseAPI.BaseAPI.RemoteCall(String TypeName, String MethodName, Objectt] Parameters, Booleann] NullList, MarshalMessageType CallType)
at SourceCode.Deployment.Management.PackageDeploymentManager.Send(String sessionName, SyncState state)
at SourceCode.Deployment.Management.SyncRequest.OnExecute(IServiceProvider provider)
at SourceCode.Deployment.Management.SyncRequest.Complete(IServiceProvider provider)
 

Diagnoses

This has to do with an unsupported Microsoft .NET version installed.
 

Resolution

Have a look at our K2 Blackpearl Support Matrix for supported Microsoft .NET versions:
http://help.k2.com/blackpearl/support-matrix_NET

Supported version for K2 4.6.9 is .NET 4.5

According to this Microsoft article: https://msdn.microsoft.com/en-us/library/ee942965(v=vs.110).aspx - you can scroll down to a section that is called "Detecting the .NET Framework". According to that section, you can see which .NET versions are installed by opening the Registry Editor on the K2 server and browsing to this directory:
HKEY_LOCAL_MACHINESOFTWAREMicrosoftNET Framework SetupNDPv4Full
Check for a DWORD value named Release. Identify which version of .Net is installed and which (if any) Microsoft KBs (updates).

The DWORD showed .NET 4.6 which is not supported.