We need more information to understand what happened. If you can't get the error details anymore, please let us know at least in which server the sub-forum was (n2.nabble.com, n3.nabble.com or n4.nabble.com?).
I see the problem. You are trying to merge the subcategory with the parent, right? The merge option deletes the subcategory and moves all topics to the parent app. This can only be done by the owner of the subcategory (because it involves deletion), in this case lowflyer1951. This option should be hidden for you and we will fix this bug. Please ask lowflyer1951 to do this for you.