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:
<forms name=".ASPXAUTH" protection="All" timeout="60" requireSSL="false"/>
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