During my upgrade from Edgy to Feisty, the upgrade program that update-manager crashed. It was in the process of installing packages, although I wasn't watching it carefully enough to know which package was being installed. All of a sudden the upgrade window vanished without a trace. There was no error message, the exit status was 0, and and nothing was printed to stdout.
This means that I don't think that it's possible to debug the crash. There should be error messages when a program crashes.
If it's any help, when I re-ran update-manager, some package kept causing a segfault with cupsys. I thought that it was the cupsys package, but closer inspection revealed cupsys to be already installed and upgraded.
Really though, this bug is more about the crash handling setup than my specific crash.
Actually there is loggin mechanism: Please attach all files of your /var/log/
Cheers,
Sebastian