After reading your input I double checked the forum folders. Seems like some very old forums had redundant html files in them, which in turn were cached and messed everything up.
I still feel uneasy about that dynamically alocated cache, but at least now has space to grow without crashing.
Now we can at least wait in peace for the UBB upgrades and judge if we keep going this way.
I'd like to thank everyone for assistance, excuse myself for the harsh words and close this discussion.
Btw: a backup routine would be nice for the future...
