Client Crash after Attachment launch

Hi all,

one of my users has a strange Problem: since the Upgrade to Client 6.5.1 (german) ((with an 5.0.11? Server)) her Notes Client crashes very often (but not always) after viewing an attachment with an external viewer-application (most of the time its a PDF-File). After closing the other application the NotesClient should be the frontmost app but this application switching does not seem to work since the Notes Client hangs while windows is still OK an she can launch other apps, switch to other running apps, but she CANT use the NotesClient: frozen Screen.

I already moved her onto another freshly installed Computer: same thing; Problem follows user…

Any Ideas? Thanks in advance!