I had that once... Turned out that the PHP config on the new server was slightly different to the PHP config on the old server. So, check your server setup. The php.ini files on both servers should be the same (since this is a migration and not an upgrade).
Also, make sure ALL the Moodle files were correctly copied across to the new server.
Lastly, check your Moodledata folder permissions too. Make sure the Apache/IIS User has full read/write access to the Moodledata folder.