This keeps popping up on backup mac. Can't get totally rid of it. Make it stop.
"Safari can't identify of the website "onetag-sys.com"
(https://i.postimg.cc/Mctpt5W7/Screen-Shot-2021-10-09-at-1-30-27-PM.png) (https://postimg.cc/Mctpt5W7)
I don't use Safari on my Mac, but that URL is an advertising platform. So I assume the site you're trying to access is trying to get to its ad provider and because their certificate has expired/is expiring, Safari is flagging it.
I assume that you don't see that message when you access every site, only one (or some?).
Two things come to mind that you may already have tried. Deleting all cookies and cache for the browser and (less desirable) checking the box that says Always trust "onetag-sys.com".
Not sure that it means much, but Chrome on Mac Catalina didn't complain when I went to that site. A google search on your problem only resulted in some very old hits that said "Update Safari" :)
I finally hit "continue" out of desperation. It hasn't come back. Hope that's a good sign.