Referencing this post from the old forums:
http://ellislab.com/forums/viewthread/152098/#738603
I’m having the same problem on 1.6.9. I have a member group, either based on Super Admins, or created from scratch, that I can’t get able to edit categories. They get a “You are not authorized to access this page” error.
I have tried toggling the settings for categories within member group preferences, as well as selecting and unselecting the member group in each category group’s preferences. Nothing appears to be working.