Re: Only admin can login?
Do you have any plugins installed? If so, try disabling them and see if users can log in.
You’re running a modified template. If so, try the default template, unmodified, and see if the problem goes away.
Do you have access to server logs? If so, look for 500 errors being logged or php errors.
UPDATE: I just created a user and I see what’s happening. Not a blank screen, they’re just redirected to the login screen to log in again. Try a stock template and try disabling plugins.