That is a bizarre error, not sure what is causing it. I've been all over vBorg, vBcom, Google looking for other instances of such, and cannot find a decent explanation, especially in light of the fact that your forum is working okay.
Both your forum and PhotoPlog call in vB's global file, which in turn calls in vB's init file. The init file uses the same procedure to set $vbulletin->bf_ugp regardless, so it's weird that your forum is working but PhotoPlog is not.
One thing to try is to echo out some information, maybe it'll give a clue.
In vB's class_core.php file find:
Code:
Content visible to registered users only.
And temporarily replace with:
Code:
Content visible to registered users only.
And then call PhotoPlog from your browser.
What do you see on screen?