Forms Authentication Error: User '' does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed

I created a custom security extension following the steps listed in the Readme_Security Extension Sample. It works fine if I login as the user that is specified AdminConfiguration section of the rsreportserver.config file but if I log in as another user, I get this error: User '' does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed.  I've added the user to both System Administrator and System User roles to try to get it to work but still no luck.

Does anyone know how to fix this?

Thanks.