Hi
I had the problem after moving my site to new host. I fixed it by changing "define('DB_DRIVER', 'mysql');" to "define('DB_DRIVER', 'amysqli');" in config.php file.
See if that helps. It at least works with my web host setup. Might be different for yours.
Also, after that I was able to upgrade to 1.2.4 without a problem. Before that the upgrade failed.