In my case it turned out to be a private dissector with a change that I had forgotten that I hadn't tested. Annoyingly, I had to step/binary-chop in the debugger with breakpoints until I could step in to the crash itself (which was in the proto_register_...() for that dissector. There was an odd crash (something to do with interfaces) after fixing this, which went away after I restored the preferences, etc files. I have since tried hiding the files again but can't seem to get back to that crash.
Martin