Hi,
I had the same issue and found a solution in the forum.
So my Plesk is working again now :)
Solution:
http://kb.odin.com/en/121714
Thread:
http://forum.parallels.com/showthread.php?306386-plesk-doesnt-work-after-upgrade-from-the-latest-11-x-to-12-0-18
Maybe this is working for...