Version of FMP 12? 12.0.3 is current https://www.filemaker.com/support/downloads/?csr=support_downloads_new
Users are on your local network? or remote?
If you open the IWP page on a browser on your computer, does it do the same thing?
"the page often hangs" how often? More than once on a computer?
Accessing computers are Mac, Win, mixed? Browsers are IE, FF, Chrome, mixed?
This happens on a local or remote network even on a browser from the host machine. It also happens on a mac or PC. It typically will hang 2-3 times before successful login. I am also using FM 12.0.3.