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I don't know if this may help..but I had problems with users logging in too, the first thing was that when they tried to logon they would always get an error message reporting some Firewall issue although they never used one. The second thing was that when they browsed my website (staying logged in) they seemed to be logged out on some pages.
What I did is I changed mainfile.php and put a 'www' prefix before my website name. Then I've added this in my .htaccess:
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{http_host} ^mywebsite.com [NC]
RewriteRule ^(.*)$ http://www.mywebsite.com/$1 [R=301,L]
and this solved both error and login problems.
Mind anchors