Boy did I hit the panic button tonight when I did a server software update and suddenly everything running in a CGI-BIN folder started generating 500 errors.
I remembered this problem with earlier versions of the Plesk control panel I use, but it seems that back then it was a simple CHMOD problem, took a look and the SUEXEC group, owner and flags were all still proper.
Lots of stuff down, clock ticking, trying this and that, mild panic setting in...
Turns out the stock Apache SUEXEC isn't compatible with Plesk 8 whatsoever which is why everything was coughing up 500 errors.
Luckily for me there is a second copy of it laying around in a Plesk folder so a quick copy and everything is back running just fine.
cp /usr/local/psa/suexec/psa-suexec /usr/sbin/suexecMost of you won't have a clue what any of this Linux gibberish is about so don't worry about it but if this happens to some other nerd using Plesk out there perhaps in his moment of nerd panic he'll find my post and know how to solve the problem quickly.