Steps to reproduce

• Access your admin interface
• click on "Admin Panel/Roles"
• Click on "Editor" role link
• Select the "Users and groups using the <editor> role" tab
• click "Assign to users/groups with subtree limitation" button
• When the UDW appear, select "eZ Platform" and confirm
• On the users/group selection on UDW, select "Editors" user group and confirm the content
• click on "Comfirm selection" button

Here a notification appears with

    Role has not been assigned to "Editors": Connection error : 406. 

And I have the response

{"ErrorMessage":{"_media-type":"application\/vnd.ez.api.ErrorMessage+json","errorCode":406,"errorMessage"
:"Not Acceptable","errorDescription":"Argument '$role' is invalid: Role '3' already assigned without
 limitations"}}

Seems that, since Editors user group already has an assignment with "no limitations" related with "editors" role, when we try to add a new one with limitations, there is a conflict.

This issue was introduced after EZP-25817, since when it was closed, the sequence of steps described above was working.