I have also suffer from this issue for several days on 3 different TW6.x.
I believe that it appears to people and not to some as there is several approach, uses and understanding of the Category feature. To some what is going on in TW is quite logical for others it is not (at all).
A/ Assign in the general permission of the TW the tiki_p_modify_object_categories to the proper group.
B/ Assign in the object permissions tiki_p_remove_object and tiki_p_view_categor to the proper group.
If i do this it work fine (for me). A user from a group with correct category perm can edit a wiki page and save it.
I don't think it should work this way…
I shouldn't have to grant "Can remove objects from the category" perm and "Can see the category in a listing". I need the object to be categorized, i need a group(s) to edit it. Not to see category and certainly not to remove from a category an object.
This say... In my case (how i apprehend category) this process is far to be clear or logical.
When you "Assign permissions to category", under Permissions -> Category the warning : "You might want to also set the tiki_p_modify_object_categories permission under the tiki section" is not enough. It shoud be in red, big or even be accessible from here (Assign permissions to category).
While on an object permissions i should be able to set all what i need... it don't make sense to set one permission (the category perms for a group) in different place.
Secondly, why the hell do i need a group to see (this is the "go" visual condition) the category tab if in anyway i don't want the group to touch it ? (in some case i don't want the group to be aware of category).
This need some polishing...