It's incorrect to assume that just because a user can access the Admin CP, they can access the forum. That isn't accurate. The only difference between an Administrator and a normal user is that an Administrator has access to the Admin CP. And that's it. Technically, it's possible to have an Administrator that has zero access to anything in the front end, but can have access to do everything in the Admin CP. For all intents and purposes, while debugging this issue, you have to forget that the user is an Administrator. It's irrelevant. Even if the Administrator is a member of the "Administrative" user group, if you have ever used "Revoke" or "Never" when setting up permissions, regardless of who you are or what group you are in, that will override an "Allow" value in permissions. Using the Analyze Permissions feature that arrived in XenForo 1.2+ is the best way to debug this issue. This is NOT the same as Test Permissions. That is not an error message that the Permissions Analyzer will return. You enter a username, and you will get a page with two tabs. It might be easiest if you screenshot the output in both tabs and that should give us a general idea where the problem is.