do you have access to the MySQL error log (not the one produced by ubb.t)? In our case there never was a corrupted table, because MySQL automatically repairs the table immediately after an error occured. During the repair always one row of the user table is dropped:
030915 21:35:53 Warning: Checking table: './dbXXX/w3t_Users'
030915 21:35:53 Warning: Recovering table: './dbXXX/w3t_Users'
030915 21:35:54 Note: Found 1784 of 1785 rows when repairing './dbXXX/w3t_Users'
Just before that "Checking table" takes place, the triggering error can be found. But not in the MySQL log, this time you have to look into your ubb.t error log.
