Hi.Claws-mail does not crash anymore on my system. I suppose one of its dependencies got updated recently, because my Debian is a frequently updated testing release, but I have no real clue.
When it crashed, I had of course restarted the whole system, and it still crashed every time. I should have run a debugger then, but didn't take the time to do so.
You can close this ticket. Sorry for the noise. -- François