Hi dbernett
Let's deal with the page error first. Would you try setting your YAF application to run in ClassicMode within IIS? I suspect it is running in the default pool. Let's see if that helps with the page error.
With respect to the NOT allow guests to post, this is controlled on a forum by forum basis. Go to your admin page and select "Forums" on the left. This will open the Forum editing area. Select an individual forum and choose "Edit". This will open the Edit Forum: area. In Edit Forum:, scroll to the bottom and ensure that Guests are set as Read Only access.
Thanks
Richard