Hi there, first post!
I had the same problem you're describing here.
Update from RC5 to ST was smooth, but when I tried to get the system back online, I got that nasty "internal error" message.
As Onideia pointed out, the error actually poped up every time I tried to save the preferences.
I started checking the code ahs10 mentioned before, and finally isolated the cause of the issue in page class.config.php, lines 510-511:
if(!$errors && Email:($var)) //Make sure admin email is not also a system email.
$errors='Email already setup as system email';
It seems to be a new verification added there, that prevents the use of the same email address as admin AND system email. I was using the same email account for both purposes, and that's why it didn't let me save the preferences.
I commented those two lines (because I don't want that restriction), and everything is working great now.
Oh, and I'm not using Fantastico (I don't even know what it is)
Well, I hope this could help anyone getting this annoying error after updating the system. ;)