Admin password rejected, but only for a couple specific tasks

I am on a 2013 27” iMac running macOS 10.13.6. Everything seemed to be working just fine. I am the only user. My account has admin rights. I am able to download, install apps and updates without issue.. Today I wanted change something in my user account settings. For the first time in recent memory (while using High Sierra) my password was rejected. But only when I try to access users and groups or security&privacy. I have used the same password since Mountain Lion. I can still login, install and update without issue. At first I thought I may have been hacked because I never actually created a ROOT account. I created a root account successfully, so I am leaning less toward hacked and more toward OS error. I am trying to avoid a complete clean reinstall if possible. Looking for ideas/suggestions.