2
Here's that line from kernel/configitem.php in version 2.0.5.2:
$sql = 'SELECT * FROM '.$this->db->prefix('config');
The error means that $this->db is not defined, or is not an object.
I'd have to investigate further to find out why the error is ocurring.
Are there any other symptoms? I.e., is everything else in XOOPS working perfectly except for that single error message?