You should install the UBB.threads System Requirements Test Script and see if it detects any problems with your configuration; the error is reporting that the mysqli_pconnect function does not exist on your server, can you disable persistent connections in your config file and see if the error goes away?

the "PERSISTENT_DB_CONNECTION" line in /includes/config.inc.php should be set to '' (blank) for this test. Additionally, you can also install the mysqli_pconnect function to your server.

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