I was asking about the forumdisplay template, not forumhome.
I assume also that your navbar template is also modified & missing the security token info:
Code:
td class="vbmenu_control"><a href="forumdisplay.php?$session[sessionurl]do=markread&markreadhash=$bbuserinfo[securitytoken]" rel="nofollow">$vbphrase[mark_forums_read]</a></td>
Verify the code above in your navbar template.