I too get a blank white page with "Hacking Attempt" in the upper left hand corner of the page.
Everthing else works WELL, and the error only occurs when the "Submit Button" is clicked after taking a test (both registered and administrators get the same error); this occurs in True & False, and Multiple Choice (didn't try the other one).
Running UQM 1.2.0 on Subsilver on phpBB version 2.0.21 withOUT Cash mod activated.
I'll try to find the problem right along with you, but no guarantees. I'll report back if I can locate it. Hope you find it first!
PS. I just discovered that IF I do NOT answer any questions at all, and I click the "Submit Button" THEN it works. Of course you get a lousy ZERO score, but at least it indicates more as to where the problem is located.