I met this problem before. But the problem is resolved. I do follow all the tutorial, but can not log in, only blank page after login. What i did is added these 2 lines to /etc/squirrelmail/config.php
$plugins = 'compatibility';
$plugins = 'change_sqlpass';
I think it (The script "/usr/sbin/squirrelmail-configure" from squirrelmail 1.4.15) has a bug. It forgets to add those 2 plugins though I did do it.
Check the script yourself make sure it 's there.The problem will be gone.