That's actually quite a hard error to track down because it's misleading in the line number it gives you as it relates to sending an error message. 9 times out of 10 it's connected to a broken query in the userlogin() function.
Find this in include/functions.php:
Code:
if (!$language['BACK'])
$language['BACK']='Back';
Add this underneath:
This will make it display the error message without any formatting so that it'll give you a rough idea about what's going wrong and where.
NOTE: Once you have the error message you should comment out or remove that line.