I've seen that happen on some servers with the Spider Friendly URLs. Try turning the spider friendly URLs off, and wait whatever the limit in your who's online is. (30 minutes) etc....
If that solves it, then try changing all occurances of "PHP_SELF" to "SCRIPT_NAME" in ubbt.inc.php (backup the file first!!!!!!) and see if that allows it to work with spider friendly URLs.
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