There are several conditions that can cause a forum to be created so that the admin group does not have rights.
The solution is to go into the groups admin page, then select the permissions for the admin group.
In this page, make sure that "Allow all" is selected for both the Categories and Forums files. Then save the settings.
You should be able to access all forums as an admin now.
If that doesn't solve the problem, try restarting the webapp. It could be a problem with the in memory repository code. A restart will rebuild this.
[originally posted on jforum.net by monroe]