When creating or editing a user's role via Admin Backend, it won't save

Hi all, hoping someone can help with this. Searched around, but couldn’t find this problem.

When I create a new user or edit an existing user and try to set a Role, I get a success message “User updated.” but there is no role saved. It still says –No Role For This User– in the dropdown box. It’s simply not saving.

Anyone have any ideas – thanks much!