I've never ran into that problem, but to be fair, I've never used ob_gz on anything with a wide audience (ie, more with more than 10 ppl from my office).
If you are sure that you haven't changed anything on your side, and if you've eliminated server side glitches (ie, reboot and see if they still have the problem). I would begin trying to duplicate the issue.
Speaking generally, if you haven't changed anything, then more than likely they have. Don't listen to any vehement or pathetic protestations of innocence. Rather, steel yourself by brushing up on a couple issues of BOFH and demand that they bow down in acquiescence and promise never to modify their system again! I started out in this industry working tech support for an ISP for a year. The most important thing I learned was: Never, Ever, EVER trust end users (or feed them after midnight).