I am getting a similar problem.
Only the super admin can login and I keep getting redirected to the index.php even when I click on Settings. I can see some of the pages like individual content pages but when you click on edit you are redirected to the index.php in admin. This was working fine for about almost a month but something has changed. It is hosted on GoDaddy (not my choice) and I don't know if they changed something.
Another thing I just noticed is that it won't let me log out now.