An easy way to reproduce is to uninstall xscreensaver. I just did this accidentially after not paying much attention to dist-upgrade & autoremove and suddenly found xlock to exit immediately after startup.
Adding to the confusion, '-mode blank' did not work since Xressources seem to override it. xlock should fall back to blank. And regardless of such failure, it should never just exit/crash. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org