When the IP address is used for the URL instead of the FQDN it causes this error: "Could not log on with the information supplied. Please try again" to be thrown.
With a form authentication, the session is stored in an HTTP cookie and sent to ISA with each request from the browser. When you try to use an IP address to access your resources, FlexAuth is trying to set a cookie for your FQDN (fully qualified domain name) but your browser won't return it in the next request. Hence FlexAuth will never be able to tell that you have logged in after the first request.
To solve this problem, always access the site with the correct host name, even when testing. You can set an entry in the hosts file if necessary, as long as the browser believes it is going to the correct URL.
In the realm settings for FlexAuth make sure the URL you enter matches the URL you are trying to access in the browser. Don't use *.example.com in the settings if your site is actually published to the Internet as something.example.NET, etc.