I had the same issue..
After further investigation from the details given in this post, I tracked down the problem as being an underscores '_' in the database keyword name for the particular forum.
Fixes are either remove and do not use underscores in forum identification names or, as I did make a change in ubbt.inc.php.
I made the change in the file and all is now accessible.
The routine to change is :
Code
ubbt_validate($validate="",$value="") {
..
..
..
elseif ($validate == "alphanum") {
        $value = preg_replace("/[^A-Za-z0-9\-]/","",$value);   <- original
        $value = preg_replace("/[^A-Za-z0-9\-_]/","",$value);  <- changed