Thanks protovu, ultimately it was good advice to reach out to tech support, which I eventually did, but not before trying everything else I could think of in frustration. I even bought a subscription rental for Studio for a month, and even that didn't work. I thought 'hey it's only 20 dollars, let's see what happens..' but I wasn't thinking clearly, since it's actually 20 euros, which is more expensive, so that was a boneheaded move that wasted a bit of money for no good reason.
Tech support was very good, they asked me to provide screenshots of my Licenses page, my Subscriptions page, and the error message I was receiving. Then they pointed out that my licenses were for Studio version and not the Enterprise version (the license page didn't show me that, at least not clearly where I could see) and I had managed to download the Enterprise version for Carrara. When I initially saw that I had been deactivated on my machine (which I'm guessing happens periodically) I figured I might as well upgrade to the latest version to see what was new and play around with it. Because it's not super clear on the downloads page what the differences are, I accidentally grabbed the Enterprise upgrade instead of the Studio upgrade. Anyway it was easily solved once tech support told me to go download the Studio version, now it appears to work just fine. So ultimately it was my own blunder that caused this
I know myself pretty well, and I'm likely to repeat this mistake again in the future, so that's why I'm writing the full version of events so I can just re-read my post when I inevitably screw up like this again.
Now that it's working, going to have a look-see and play around to see what's new...