You need to update your templates. Their was a fix in 6.04f. You need to find this line in
public_pm_forum.pl :
<FORM NAME="REPLIER" ACTION="$vars_config{CGIURL}/ultimatebb.cgi">
And change it to:
<FORM NAME="REPLIER" METHOD="POST" ACTION="$vars_config{CGIURL}/ultimatebb.cgi">