SYNC between Outlook and Nextcloud calendar failed

Since a few weeks the sync between Outlook (Microsoft Office Professional Plus 2021) and Nextcloud via CalDavSynchronizer (Version 4.3.0.0) doesn’t work always. Mostly I get an error message (see below).
Connection test is successfull.

DAV URL: https://nextcloud03.webo.hosting/remote.php/dav/calendars/fam.burkhardt@online.de/burkhardt-family/

User: fam.burkhardt@vodafone.de

CalDavSynchronizer.DataAccess.WebDavClientException: Fehler beim Senden der Anforderung. —> System.Net.Http.HttpRequestException: Fehler beim Senden der Anforderung. —> System.Net.WebException: Die zugrunde liegende Verbindung wurde geschlossen: Eine Verbindung, deren Aufrechterhaltung erwartet wurde, ist vom Server geschlossen worden…
at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)
— End of inner exception stack trace —
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at CalDavSynchronizer.DataAccess.HttpClientBasedClient.WebDavClient.<ExecuteWebDavRequest>d__9.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 CalDavSynchronizer.DataAccess.HttpClientBasedClient.WebDavClient.<ExecuteWebDavRequestAndReadResponse>d__7.MoveNext()
— End of inner exception stack trace —
at CalDavSynchronizer.DataAccess.HttpClientBasedClient.WebDavClient.<ExecuteWebDavRequestAndReadResponse>d__7.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 CalDavSynchronizer.DataAccess.CalDavDataAccess.<GetVersions>d__29.MoveNext()
— End of stack trace from previous location where exception was thrown —
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at CalDavSynchronizer.DataAccess.CalDavDataAccess.<GetVersions>d__29.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 CalDavSynchronizer.Implementation.CalDavRepository1.&lt;GetAllVersions&gt;d__9.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 GenSync.EntityRepositories.VersionAwareToStateAwareEntityRepositoryAdapter4.<GetFullRepositoryState>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 GenSync.Synchronization.Synchronizer11.&lt;Synchronize&gt;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 GenSync.Synchronization.ContextCreatingSynchronizerDecorator7.<Synchronize>d__3.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 CalDavSynchronizer.Scheduling.SynchronizationProfileRunner.<RunAndRescheduleNoThrow>d__22.MoveNext()
false
00:00:00.3473652