What WP version are you running? Same results if you disable other plugins? Same symptoms for all post types (post/page/custom)?

Does the post have private visibility? What are the WordPress roles of the other users? Have you modified role definitions or set Restrictions?

I'm having trouble reproducing the lonely users dropdown based on those config details. Is this a multisite installation?

Make sure Roles > Options > RS Role Defs > Post Author doesn't show any warning message about mismatch with WP role caps.

The role-scoper information at the bottom of a affected post (which belongs to admin at the moment) shows "Readers" and "Contributors" rights for the desired groups and users - in contrast the "Editors" section only lists admin as allowed… but this looks identical to the situation of an post which was initiated by a user himself…

Another strange fact:

User A and B are in a group

The group has rights for a category

User A has additionally Rights for subcategory thereof

I log in as User A and create a new post - only the appropriate category and subcategory are shown (as it should be) I choose both and publish the post.

Now I log in as admin and edit the post, the box for author now contains all members of the group but not other users which do not belong to it (again - exactly what we want)

Next step… I change the author to User B and still the world is ok…

But now finally… as soon as I change the author again and this time to admin and thus usurping the post all other possible choices in the author dropdown box are gone… even though the categories of the post never changed at all throughout the whole process.

It seems as if a post which belongs to admin always has to stay as such but one which belongs to a user may still be distributed to other users within the same group.

Last but not least I granted editor rights for the posting to a user but admin is still the only choosable author.

It sounds like some other plugin is applying additional filtering to the Post Author dropdown when an Administrator is the current author.

I think I just found out when the problem disappeared and I can make it reappear… last time Kevin offered to take a look at the blog and in preparation I added a second account with administrative rights - today I cleaned it up and removed the account and right away the problem with the author selection box was back and after adding a new administrator account it disappeared again.

I will leave a second dummy-administrator account active as a workaround but perhaps this information helps to detect the underlying problem in the future.

If only one administrator exists, the dropdown only contains him and noone else.

If at least two administrator accounts exists, the dropdown shows not only all administrators but additionally all the other users. If a category is chosen for the post the displayed users in the dropdown are filtered according to the rolescoper-configurations only listing the users with access to the category.

Taking the last fact into account I assume it is proven, that rolescoper does indeed its job and filters the dropdown… the question remains why a second admin account has to exist to push it into doing so?

Is it possible that somewhere a shortcut has been implemented by wordpress or rolescoper which only triggers the actual processing of the rolescoper-rules if "more than one" user is eligible for post author according to plain basic wordpress role definitions?