I managed to hose my Piwigo installation some time ago trying to get it up to 2.7. For some reason, it never completely went through, so for the longest time, I have a Piwigo instance that was half 2.7 and half an earlier version. (Doesn't make much sense to me, either, but there you are.)
Multiple attempts to upgrade it to 2.9 only resulted in tons of SQL errors and rendered it completely unusable. After a lot of homework and experimentation in localhost, I think I got it. The fact that there is finally a new album and a new photo here speaks to the power of persistence.