Hi,
evtl. ist das die Ursache:
With all Notes Releases: When you have a handle to a parent document and mistakenly attempt to use ParentDocumentUNID with the GetDocumentByUNID method, the following error occurs:
"Invalid universal ID"
The error code associated with this message is 4091.
It is important to note that this is the same error message you receive when you attempt to access the parent of an orphan in Notes releases 4.6, 4.6a, and 4.6.2x.
One way to avoid trying to access the parent of a document that is actually a parent itself is to use the IsResponse property (of the NotesDocument class). This methodology is utilized in the Conclusion section of the technote.
Andreas