There is a bug in your code: where you try to change a role for a user in the back-end, nothing happened. But a record with \"1 0 1 NULL\" generated in the table. That cause the \"You are not allowed to view this page!\" message for all public users since then. Please check.
Another issue: where a visitor choose a role from the front-end, he will be re-directed to the front page. I believe the itemid=1 parameter caused this problem if the job menu item id is not 1 in our Joomla setup.