- Most of them have either wrongly configured Form Based Authentication and on proper configuration it started to work.
- For few the Claims to Windows Token had stopped, and on restart of the services FBA started working.
- Few also suggested the performing an IISRESET on the server solved their issues.
- In some case the APP Pool Credentials for Security token services was wrong.
Hope this post would help anyone facing similar issue. Please feel free to drop any comment or any other alternate approach.