I had this problem as well. There is another "Mark Forums Read" link in the FORUMHOME template, which was the main one I was using.
I replaced (adding the markreadhash)
Code:
<a href="forumdisplay.php?$session[sessionurl]do=markread" rel="nofollow">$vbphrase[mark_forums_read]</a>
with
Code:
<a href="forumdisplay.php?$session[sessionurl]do=markread&markreadhash=$bbuserinfo[securitytoken]" rel="nofollow">$vbphrase[mark_forums_read]</a>
and it worked like a charm. Thanks!