Users cannot login after Upgrade to 7
To figure out what was the problem, thanks to this post I opened include/error.inc

and replaced

print theme('maintenance_page', array('content' =&gt; t('The website encountered an unexpected error. Please try again later.')));


with 

print theme('maintenance_page', array('content' =&gt; t('The website encountered an unexpected error. Please try again later.'.$fatal.' '.var_export($error,true))));


The new command prints out some useful info that can be used to debug. In my case, it turned out that messaging module caused the problem. I disabled messaging, and could login afterwards.

Note: before going to production, switch back to original error.inc, since the error outputs can be a gift to attackers.