This is quite likely caused by the logged in user not being a member of the Administrators role...
You may want to compare your membership/roleprovider configuration between the working site and the non-working site.
Also, this may be helpful if you are using a windows membership provider: http://blog.tomstenius.com/2009/04/episerver-community-role-and-membership.html
We have an instance of CMS 6.0. I need to install Community for one?
I have tried install Community on CMS 5.0, then upgrade one to CMS 6.0. It works perfectly. Then I merged all changes in web.config to desired instance CMS 6.0 and added a record to connectionstring.config on existing Community's db. My site works, but I see "Access is denied" message for any item from Community tab in Edit Mode. What is wrong? How can I fix that?