Novell GroupWise 2014 R2
If a user tried to send a mail, after clicking on Send he was logged off his mailbox.
There was a problem with a signature of that user. Within a signature there was a picture which was actually attachment (a picture) of some mail which was viewed. If a user used a native GW client to edit the signature, accept a certificate prompt popped up. After correcting the picture (no more referencing to old mail attachment), the problem disappeared.