Strange login problem, logging in bit then showing as guest

[RDB]

hi,

recently o moved my forum from whangout.com to Webmaster Forum, Web Hosting Forum, SEO Forum, Internet Marketing Forum, Page Rank Updates, SEO
but after that I am facing strange problem.

when someone try to login it shows as guest after redirection.

try this:

user: test
pass: test

even on mobile this problem is more complex. as when I try to login it keep showing me that same page........

I tried everything. I uploaded all non image files again, checked htaccess file bit the problem os not getting solved.

can someone help me on it please.

 

[RDB]

I have following htaccess code :

# Comment the following line (add '#' at the beginning)
# to disable mod_rewrite functions.
# Please note: you still need to disable the hack in
# the vBSEO control panel to stop url rewrites.

# Some servers require the Rewritebase directive to be
# enabled (remove '#' at the beginning to activate)
# Please note: when enabled, you must include the path
# to your root vB folder (i.e. RewriteBase /forums/)

RewriteEngine on

#RewriteBase /

RewriteCond %{HTTP_HOST} ^webmasterforumuk\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.webmasterforumuk\.com$
RewriteRule ^/?$ "http\:\/\/webmasterforumuk\.com\/" [R=301,L]

RewriteRule ^((urllist|sitemap_).*\.(xml|txt)(\.gz)?)$ vbseo_sitemap/vbseo_getsitemap.php?sitemap=$1 [L]

RewriteCond %{REQUEST_URI} !(admincp/|modcp/|cron|vbseo_sitemap|api\.php)
RewriteRule ^((archive/)?(.*\.php(/.*)?))$ vbseo.php [L,QSA]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !/(admincp|modcp|clientscript|cpstyles|images)/
RewriteRule ^(.+)$ vbseo.php [L,QSA]

And the site showing this message in firefox:

The page isn't redirecting properly

Firefox has detected that the server is redirecting the request for this address in a way that will never complete.

    This problem can sometimes be caused by disabling or refusing to accept cookies.

Please help someone...