if you configured your SharePont Site to use Forms Authentication,you might go through a very strange problem,when you open any page inside your sharepoint site,you will a white page showing “File Not Found” message,and this is a stopper problem,you can’t configure your site or work with it at all.
I tried to find the solution and it’s really tricky problem,if you updated the web.cofig try to open it now,you will find this attribute in web.config:
what you have to do:
1) Remove xmlns attribute in web.config
2) reset the IIS.
navigate to your sharepoint site,it’s working fine.
happy times with MOSS :).
Hope this helps.
Ref. Read More :