You load your login screen on your IPhone, enter the Username and Password and click to Log on, but the login screen is being reloaded and it doesn't log you in.
Make sure that your IPhone is set to accept cookies. If your IPhone is set to not accept cookies then you will see this behaviour.
To turn your cookies on in your IPhone:
Go to 'Settings' - 'Safari'
Select 'Accept Cookies' and set this to "From Visited" or "Always".