Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I solved this issue for myself by doing this:
Open Internet Explorer (I know, weird, right? Apparently they are connected somehow and even if you never use Explorer you have to do this to solve this activation problem.)
Go to Tools>Internet Options>Advanced>Reset Button
Once it has completed resetting to default settings, it will prompt you to restart your machine.
Then Office automatically started streaming..for quite a while..presumably an additional update??
When that was complete, I opened Excel, clicked Activate and it worked. It ACTUALLY worked, after a week of "Sorry, we are having temporary server issues." Turns out it wasn't temporary OR a server issue. I hope this works for everyone with this problem and you won't have to spend hours on the phone and Googling for clues to troubleshoot this error.