Thanks a lot, Leki. I have found these rewrite rules in the .htacces file, and one of them could be the key of the error:

RewriteRule ^featured/(.*) video.php?category=featured&page=$1
RewriteRule ^random/(.*) video.php?category=random&page=$1
RewriteRule ^rss/new/ rss.php?type=new [L]
RewriteRule ^rss/comments/ rss.php?type=comments [L]
RewriteRule ^rss/views/ rss.php?type=views [L]
RewriteRule ^group/(.*)/ groups_home.php?urlkey=$1

Or i remember that at the begging there was a user called vShare (the first one) and I deleted it. It could be that one the reason of the problem because it could be a group created by the first user and when deleting now the link is broken. Should I go to the datebase to create a group or to rewrite the first user? Or where can i get the updated htaccess file for the version 2.5?