I could reproduce this by opening two crypto mails in multiple windows this
reliably triggered the crash.

I have not fully understood the crash as it crashed in the close invocation in
outlook. After various trys and improvements to our code (there were some fishy
cleanups) i was able to fix this by closing the inspector of the mailobject
before closing the mail. Outlook apprarently did not like it if I closed a mail
that was active in an inspector but that is a bit speculation.