This can occur due to incorrect authentication settings in the website configuration files.  To check them:

1.  On the NetCommunity webserver, edit the web.config file in the NetCommunity directory.  The default path is "C:\Program Files\Blackbaud\NetCommunity\web.config"
2.  Search the file for "authentication mode"
3. Check the <authentication mode="Forms"> section to make sure it is correct.  It should look similar to the section below:

<authentication mode="Forms">
   <forms name=".ASPXAUTH" protection="All" timeout="60" requireSSL="false"/>