Attempt To Perform Folder Operation On Non-Folder Note

I updated a Notes 7.0.3 server to the new 8.0.1 release. All mail-files were converted to ODS 48 using compact -c “mail-file”.
Everything works fine until I started our Blackberry Enterprise Server which is installed on a separate machine and accesses the mailfiles of 25 users on the recently updated server.
When the BES accesses a mailfile, an error message occurs on the Domino server console:

22.03.2008 13:17:24 Database note open error: NT00000566 document in database g:
ed by CN=BLACKBERRY/O=XXXXXX: Attempt to perform folder operation on non-folder note.

This error looks similar to the one described here and here. In both cases the error occured because a non-folder notes contained an $Name item with the value ($Inbox).
I investigated the mail files in question but could not find any duplicate note.

All Note-IDs in the error messages refer to a unique design element in all affected mail files: ($Sent).

This is a non-folder note, indeed. I did another search for duplicate notes containing ($Sent) in the $Names field, but there are no duplicates … My next attempt was to delete the ($Sent) view and replace the design with the template.

But even this has no effect; the error still occurs, although the sychronization between Domino and the Blackberry devices via BES seems to work without any error.

Well, I think that I’ll have to create a service desk call at IBM. Or can anyone of the Blackberry gurus within the Notes Community shed a light on this issue ?