Content type already exists when activating site collection feature

We have installed Nintex Workflow 3.1.4.0 and Nintex Forms 2.6.0.0 on an fresh SharePoint 2013 farm.

When it comes to active the "Nintex Forms for Nintex Workflow" we get the following error message (sorry for the German).

Der Inhaltstyp mit der ID '0x0108010079DBDE612F7B46928C6A2516BA2CAE3700D4A837248A47E040ABD1A569613E898B', der im Feature '{ac8addc7-7252-4136-8dcb-9887a277ae2c}' definiert ist, wurde in der aktuellen Websitesammlung oder in einer Unterwebsite gefunden.

But according to the ULS log, the xml definition cannot be located:

Unable to locate the xml-definition for CType with SPContentTypeId '0x0108010079DBDE612F7B46928C6A2516BA2CAE3700D4A837248A47E040ABD1A569613E898B', exception: Microsoft.SharePoint.SPException: Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED)) ---> System.Runtime.InteropServices.COMException: Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))

at Microsoft.SharePoint.Library.SPRequestInternalClass.GetGlobalContentTypeXml(String bstrUrl, Int32 type, UInt32 lcid, Object varIdBytes)

at Microsoft.SharePoint.Library.SPRequest.GetGlobalContentTypeXml(String bstrUrl, Int32 type, UInt32 lcid, Object varIdBytes)

--- End of inner exception stack trace ---

at Microsoft.SharePoint.SPGlobal.HandleComException(COMException comEx)

at MicrosoftSharePoint.Library.SPRequest.GetGlobalContentTypeXml(String bstrUrl, Int32 type, UInt32 lcid, Object varIdBytes)

at Microsoft.SharePoint.SPContentTypeCollection.FillCollection(SPRequest req, SqlDataReader rdr, Boolean openWeb)

Leaving Monitored Scope (Feature Activation: Activating Feature 'NintexFormsWorkflowSite' (ID: 'ac8addc7-7252-4136-8dcb-9887a277ae2c')

at URL http://webshape.test.hatebur-net.local.). Execution Time=164,840084424138

Application error when access /_layouts/15/ManageFeatures.aspx, Error=Der Inhaltstyp mit der ID '0x0108010079DBDE612F7B46928C6A2516BA2CAE3700D4A837248A47E040ABD1A569613E898B', der im Feature '{ac8addc7-7252-4136-8dcb-9887a277ae2c}' definiert ist, wurde in der aktuellen Websitesammlung oder in einer Unterwebsite gefunden.

at Microsoft.SharePoint.Utilities.SPUtility.ThrowSPExceptionWithTraceTag(UInt32 tagId, ULSCat traceCategory, String resourceId, Objectc] resourceArgs)

at Microsoft.SharePoint.Administration.SPElementDefinitionCollection.ProvisionFieldsAndContentTypes(SPFeaturePropertyCollection props, SPSite site, SPWeb web, SPFeatureActivateFlags activateFlags, Boolean fForce)

at Microsoft.SharePoint.Administration.SPElementDefinitionCollection.ProvisionElements(SPFeaturePropertyCollection props, SPWebApplication webapp, SPSite site, SPWeb web, SPFeatureActivateFlags activateFlags, Boolean fForce)

at Microsoft.SharePoint.SPFeature.Activate(SPSite siteParent, SPWeb webParent, SPFeaturePropertyCollection props, SPFeatureActivateFlags activateFlags, Boolean fForce)

at Microsoft.SharePoint.SPFeatureCollection.AddInternal(SPFeatureDefinition featdef, Version version, SPFeaturePropertyCollection properties, SPFeatureActivateFlags activateFlags, Boolean force, Boolean fMarkOnly)

at Microsoft.SharePoint.SPFeatureCollection.AddInternalWithName(Guid featureId, Int32 compatibilityLevel, String featureName, Version version, SPFeaturePropertyCollection properties, SPFeatureActivateFlags activateFlags, Boolean force, Boolean fMarkOnly, Boolean fIgnoreMissing, SPFeatureDefinitionScope featdefScope)

at Microsoft.SharePoint.WebControls.FeatureActivator.ActivateFeature(Guid featid, Int32 compatibilityLevel, SPFeatureDefinitionScope featdefScope)

at Microsoft.SharePoint.WebControls.FeatureActivatorItem.ToggleFeatureActivation()

at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)

at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Microsoft.SharePoint.SPException: Der Inhaltstyp mit der ID '0x0108010079DBDE612F7B46928C6A2516BA2CAE3700D4A837248A47E040ABD1A569613E898B', der im Feature '{ac8addc7-7252-4136-8dcb-9887a277ae2c}' definiert ist, wurde in der aktuellen Websitesammlung oder in einer Unterwebsite gefunden.