Both links function flawlessly here.  In fact, the file concerned, admin_ug_auth.php is unchanged between Standard DF forums and ForumsPlus.  I can only surmise  that somehow ForumsPro is causing some interference, or that you have some missing table(s).
Forums, ForumsPlus and Forumspro all use the same tables:
cms_bbuser_group
cms_bbgroups
The difference comes when info is saved:
ForumsPlus updates cms_bbauth_access
ForumsPro updates cms_fpro_auth_access
That difference is exactly why they should happily work simultaneously.
As I state to people, ForumsPlus is simply an upgrade:
- did you first have Forums installed before adding ForumsPlus?
 
- did you then enter modules admin and click "Edit" to upgrade the installation?
 
- do you get any sort of debug error info?
 
- I can't help but feel that you are missing some part of the forum database table structure