Troubleshooting the MySQL or Apache servers themselves is out of the scope of forum support; UBB.threads is a simple PHP/MySQL script which just runs along side of the database and web servers themselves, all it attempts to do is create a connection to the server based on the server information you provided via setup or can change in the /includes/config.inc.php file.
What I would suggest is clearing ample space on the machine and restart the MySQL server and run a repair on the tables in the UBB.threads database.
