API: Problems with Search

When calling portal/shop/serviceitems?..&search=, from time to time an error occurs that we cannot reproduce/explain

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If I compare a successful and an unsuccessful call, one can notice the position where this happens.
On success, the WebLog shows Filter strategy for AccProduct --> Index (AccProduct), but in case of an error the following message is provided at the very same position (everything logged bevore is equal):

System.Exception: An error occurred while processing the request: GET https://.../portal/shop/serviceitems?UID_Person=2febbe7c-429e-4d37-b003-70c2b0f94e83&OrderBy=Ident_AccProduct asc&StartIndex=0&PageSize=20&UsePeerGroupFilter=false&UID_PersonPeerGroup=2febbe7c-429e-4d37-b003-70c2b0f94e83&IncludeChildCategories=true&search=test
---> VI.Base.ViException: Sie haben keine Berechtigung für diese Aktion. Melden Sie sich an.
at VI.DB.Implementation.Connection._CheckSession()
at VI.DB.Implementation.Connection.get_Session()
at QBM.CompositionApi.AspNetConfig.<.ctor>b__0_2()
at QBM.CompositionApi.AspNetConfig.CheckingConnection.<>c__DisplayClass0_0.<.ctor>b__0()
at System.Lazy`1.CreateValue()
at System.Lazy`1.LazyInitValue()
at VI.Base.ResetLazy`1.get_Value()
at QBM.CompositionApi.AspNetConfig.CheckingConnection.<CheckConnectionAsync>d__10.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Handling.EntityCollectionRequest`1.<>c__DisplayClass72_0.<<BuildFilterStrategyAsync>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.FilterStrategy.SearchFilterStrategyBuilder.<GetInternalAsync>d__27.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.FilterStrategy.SearchFilterStrategyBuilder.<GetAsync>d__26.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Handling.EntityCollectionRequest`1.<BuildFilterStrategyAsync>d__72.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Handling.EntityCollectionRequest`1.<GetDataSourceAsync>d__68.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Handling.EntityCollectionRequest`1.<GetEntityCollectionDataAsync>d__64.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Definition.EntityMethodRouteBuilder.<>c__DisplayClass20_0.<<BuildResponseBuilder>b__2>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.ApiManager.JsonResponseBuilder.InnerJsonResponseBuilder.<WriteAsync>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.Compression.CompressionResponseBuilder.CompressedResponse.<WriteAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at QBM.CompositionApi.ApiManager.MethodRequestHandler.<SendAsync>d__12.MoveNext()
--- End of inner exception stack trace ---


Since all the other actions still work (I am obviously logged in) and the search is carried out via the AppServer, I understand this to mean that the login to the AppServer is no longer valid. However, I have not found any information in the log files on the AppServer.

Any ideas what else I can do?