Hallo Wilfried,
you don't have to be sorry for don't doing things that are not really your business! So this here should be more a short hint for those who are interested in as an answer...
The main cleaning up doesn't belong to the OnDisconnection method of the IDTExtensibility2 interface but to the OnBeginShutDown method.
That is at least the point. If done everything what belongs to the addin is finished when the MapPoint application is starting its unloading. No further problems.

Thank you for care
thomas martin