Is it a popup form?
I would use your Firefox or Chrome developer tools and follow the "bread crumbs" for lack of a better term LOL.
Or, look at your NT server's log and see pretty much the same activity. (The dev tools are easier for me personally).
Observe the activity in the devl tools when you click to "login." You should see your POST being processed. You see be able to see your login credentials as passed parameters. Double check that your login form is doing what it's supposed to.
Are you processing the login in the login form or in the WebHandler? Either way, double check that your session values are getting set. If you're not already, I highly recommend moving the login processng to the WebHandler.
Under the Project Properties, Compiling tab, set NetShowSend and NetShowReceive to 1. Then you can observe everything NetTalk send and receives in DebugView. Pay close attention to you Request and Response headers and the passed parameters.
There were no changes, according to the NT history, that should have affected the login.
If you still cannot get in, shoot me an email and I would be glad to remote in and take a look at what's going on.
donaldridley2011@gmail.com
Best of luck,
Don