In CRM/Core/COnfig.php the reference to Joomla user table has a hard coded prefix

In CRM/Core/COnfig.php line 272 the reference to Joomla user table has a hard coded prefix of jos, which is the default but not required (and multisite installs often use separate prefixes for each site.

The value for prefix can be obtainded using getConfig

$dbPrefix = $mainframe -> getCfg( 'dbprefix' );