BadRequest/Connection cannot be reused when migrating inside OnPremise Instance

[chripf]

When migrating work items from one collection to a different one, but on the same server, migration fails with

Authentication failed because the connection could not be reused.

• I am using Integrated Auth for both connections
• User has rights on both.
• 4 work items were migrated, now every approach fails with the error above.
• Azure DevOps Server 2020 Update 1

Stacktrace:

   at Common.ApiWrappers.ApiWrapperHelpers.<ExecuteBatchRequest>d__3.MoveNext() in D:\Sources\vsts-work-item-migrator\Common\ApiWrappers\ApiWrapperHelpers.cs:line 27
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Common.ApiWrappers.BaseBatchApiWrapper.<>c__DisplayClass2_1.<<ExecuteWitBatchRequests>b__2>d.MoveNext() in D:\Sources\vsts-work-item-migrator\Common\ApiWrappers\BaseBatchApiWrapper.cs:line 42
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Common.RetryHelper.<RetryAsync>d__4`1.MoveNext() in D:\Sources\vsts-work-item-migrator\Common\RetryHelper.cs:line 31