Dunno if this'll help much, but I solved my problem in this thread, and many others, by doublechecking mods I'd made to the core code. GL is generally really great for tweaking, 'specially if you don't "know" PHP, but as soon as you get outside the config.php file, and the templates, you really gotta pay attention to what you leave in, change, remove... Quite often, little mistakes don't break the program, they just have odd effects. And, though the templates are pretty good, there's still some stuff that you have to go into the code to change (like, for example, the default stuff on the top menu bar, "polls", "links", "advance search", etc).
Though I often don't bother, a good common sense 3-step approach to safely making any changes is, starting from an installation that's working fine:
1) make a BACKUP COPY of each file you're gonna modify, BEFORE touching it;
2) keep track of what you did by COMMENTING IN THE CODE where the change is made, y'know:
Text Formatted Code
// MYCHANGE: this is what I did right here
and also, especially if you're making lots of mods, keeping a Changelog, a list of what you did where, basically, copy your MYCHANGE notes with filename and location (eg: line number).
By commenting with a standard unique tag like MYCHANGE, you can use any text "find in file" routine (they're in lots of text editors and standalone utilities) and find all of your changes in all program files in a few seconds. (This is good for replicating your mods after upgrading GL.)
3) (Especially with lots of changes) check your progress by running GL after each mod to make sure it STILL WORKS, to catch errors early.
Doing the above actually becomes fairly automatic and doesn't take much time at all. With that stuff, you can always step back from an error to a working version by undoing your recent changes (and in extremes, roll back to the backup files that definitely worked). And it's real easy to zone in on where the problem may be when hitting a forum like this!
Also, this system definitely comes in handy when installing integrations, like Gallery, as a backup to what's in the add-on installation instructions.
It's amazing how many things are easily troubleshot by having a clean, easy backtrack route!!
(This is partly me writing a 100 times on the blackboard that I should ALWAYS do the above as well...) Hope it helps...!