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Could you post your mainfile.php, (edit out your passwords etc)
I would put it back the way it was if that did not work. If your XOOPS is on the same server and it was working ok your server config should be compatible.
By server config I meant your server type, php version, and sql version.
Also if you turn on debug in system admin/preferences/general preferences and post the results this would help.
Another double check is to do an update of your modules in system admin/modules. If your site has no content perhaps do an uninstall of the modules and re-install.
I looked through the posts on this subject and cant see any obvious answers. A last resort is to delete your database and do a fresh install, including uploading the files on your server.
Someone else had problems like this because of a "temporary path" to the domain server when initially transfered