There is no such file as ubbthreadsphp.inc.php. That file does not exist at all.
It almost looks like you have some sort of .htaccess file causing the ubbthreads.php to interpret /ubb/ubbthreads.php as the transaction and it is trying to load ubbthreadsphp.inc.php which does not exist. I have looked at the code in ubbthreads.php a number of times and I cannot find how that behavior would be caused by the scripts.
Out of curiosity, can you try to run this in /forums instead of /ubb and see if you are able to duplicate the issue? If you are able to duplicate the issue, there might be something in a lib I'm missing, a problem with your PHP config or a rogue .htaccess file on your server.
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