Yep, that worked for me too. Go to Settings and then Safari (in the left column on an iPad). Click on Clear Cookies and Data. My Wi-Fi hotspot logins were intermittent, sometimes working, sometimes not. The logins were always really slow even when they worked. When they failed, either the AP/router login screen would fail to appear at all, or it would time out with the error "Hotspot login could not open the page because the server stopped responding."
Why does this work? It isn't digital voodoo. The system applet uses Safari to determine whether a login is required. (It tries to reach a page at apple.com - if it can't do it, then a login is required.) When a login is required, the system applet again uses Safari to display the login page for you. Corrupt or incorrect data in the cache can prevent either of those processes from happening correctly.
Other things to do concurrently to maximize your success:
- Reboot the router if it is in your control. (If the iOS device and the router have different ideas about the need for the device to supply credentials, this will put everybody on the same page.)
- Close all Safari tabs before you clear the cache (bookmark the pages if you need to save them)
- Force quit Safari before you clear the cache
- Force quit all other apps before you clear the cache
- Force quit the System app after you have cleared the cache
- Hard boot the iOS device. (Power off by holding the on-off button until the red slider appears, then use the slider, then turn it on by holding down both the on-off button and home button until the white Apple logo blinks.)
All of the above will help insure that there are no other processes getting in the way of a clean cache and a fresh start.