Craft 2: Suspend user results in "Unauthorized You don’t have the proper credentials to access this page." This pertains to a Craft v2.9.2 installation.
User X is an admin user. So she has permission for pretty much everything, except accessing site settings, etc.
For an unknown reason, User X is unable to suspend other users.
User X has all permissions assignable under General and Users on the Permissions tab.
Yet when she tries to suspend another user, Craft returns the following error:
Unauthorized
You don’t have the proper credentials to access this page.

I have no idea why. Is there something I am over-looking regarding what's required for a user to suspect another user?
Is there a way to find out what is causing this issue? Such as a log of such issues?
Thank you.
Is she trying to suspend another user who's an admin user?
I just tested this and your scenario would happen if she's trying to suspend an Admin's account.
A user who's has Admin privileges can only suspend another Admin otherwise you'd have users with lower-level permissions overriding those with higher level ones.
In Craft 3, you get a more clear message if this happens: