As others suggested, I tried connecting through a cellular hotspot connection. If on cellular data, True Key synced without any problem. As soon as I went back to my home internet connection (AT&T fiber), the sync error came back.
Sometimes when this happens, it is a DNS problem. So I tried switching my PC DNS to point to Google's DNS servers instead of AT&T. No change - sync error.
That's very interesting: today I went through the signin process on a laptop connected through cellular, and was able to install Truekey on all three browsers and login was successful (I was able to add that login/browser combination) to Truekey. Logins were up to date also.
Going back to home internet connection, Truekey still works and is up to date, but it shows the 'sync error still. So this is good for a workaround but not a fix. I also tried ipconfig /flushdns on the laptop but that didn't change things. Maybe try the router next...
Update: I rebooted my home router and now Truekey is syncing ok on the laptop I'm working on. You can even see the 'syncing' message come up and then - no error!
I have yet to test the other machines at home but hope to get around to that later. Weird - could it be as simple as that?