To answer regarding extensions, I only have IDM Integration Module, Opera's Install Chrome Extensions and Windscribe, and that's all. But even so, Windscribe is not turned on at the time of this writing.
I wonder about the nature of Windscribe - whether it makes certain changes to the browser/system that are not fully "backed out" by merely turning it off. If those changes are detectable by website 'sniffing', then it could create an ad-blocking/user-location problem.
Okay, now I know the cause of the problem: It's Windscribe!
Even if its VPN is turned off and not running, its own adblocker is still running. I have to manually select "Allow Ads" from the extension's option. Now it's working fine.
Took me a while to spot the problem.