CMS Builder Error can't pull up Admim
This is likely an issue with the server configuration. One thing you can check is in your settings.data.php file to see if the "session_save_path" item has a value set. If it does, you can try to remove the value so that the line looks like this: (I would recommend copying the value somewhere else in case you need it later)
'session_save_path' => '',
If this does not help I would recommend getting in touch with your client's hosting provider to see if they can provide any insight.
Let me know if you have any questions.