It should be one of the poll templates.
Have a look in showthread_polloptions, and check that it has $session[dbsessionhash] (as bellow) and not $session[sessionhash].
Code:
<form action="poll.php" method="get">
<input type="hidden" name="s" value="$session[dbsessionhash]">
<input type="hidden" name="action" value="pollvote">
<input type="hidden" name="pollid" value="$pollinfo[pollid]">