nginx fastcgi
I ran into a problem using nginx and squirrelmail. Squirrelmail would display correctly but would return an error when you tried to login. It turned out to be a problem with the user running php-fastcgi. I am using a script that uses spawn-fcgi (from lighttpd) and had the user set to www-data (nginx user). Since I had previously used apache, I reset the script to user apache and ngnix to user apache.
That cleared my problem.