Try to disable Thanks plugin or (if already disabled) remove this line from the 'headerinclude' template:
<script type="text/javascript" src="jscripts/thx.js"></script>
I saw this plugin causing troubles in the past as well.
i disabled the thanks plugin but i don't have that line in 'headderinclude' and the problem persists...
You have jQuery in your forum.
In the footer template, after this line:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
Add:
<script type="text/javascript">jQuery.noConflict();</script>
i don't have the first line in footer.php
If you want I can take a look in your ACP if you PM me a login for admin account. Can be a temporary one as long I have access to templates and moderator permissions.