Sometimes, Sharepoint Sites give a File Not Found screen when using
Forms Authentication. The solution is below.

Delete culprit line in web.config

1. Navigate to root virtual directory of the culprit site
i.e. C:\Inetpub\wwwroot\wss\VirtualDirectories\8004
2. Open web.config using NOTEPAD
3. change the tag
<configuration
xmlns=schemas.microsoft.com/.NetConfiguration/v2.0>" target="_blank" rel="nofollow">schemas.microsoft.com/.NetConfiguration/v2.0>

so that it just reads
<configuration>

* delete the
xmlns=schemas.microsoft.com/.NetConfiguration/v2.0 section
from the <configuration> tag