Check your web server's error log.
The other possibility is that it's caused by an optional plugin. If you back up on one Moodle and restore on another, make sure they have the same plugins (or do not include the missing ones in the backup)
The other possibility is that it's caused by an optional plugin. If you back up on one Moodle and restore on another, make sure they have the same plugins (or do not include the missing ones in the backup)