I am adding my voice to this problem.
After researching this I am fairly confident that the problem lies in the information sent to the Unity servers from the Unity Hub when attempting to activate (manual or by key).
When I check the information attached to the seat I am trying to activate it does not match my system at all.
It does not detect my system correctly and identifies my system as 32 bit, it is a 64 bit x299 workstation. My Unity version is also identified as a 2017 version, I have only used Unity for a few months.
As a result a system trying to authenticate fails as the system specs wont at all match what is bound to the seat.,I am going to add my voice to the list of people having this issue.
It appears that the problem is being caused by Unity Hub sending incorrect data to the Unity servers regarding hardware configuration.
When I check my seat information it lists the following information incorrectly:
32Bit → I am on 64 bit 1909
N/A computer model → I am on a X299 Apex
Unity version 2017.2.0 → I am on 2019.3.3
Uninstalling all of Unity including manual file and registry remnant cleanup followed by reinstall does not correct the issue.
Support does not seem to be aware of this problem yet google seems to indicate that Unity is well aware.
FYI, manual activation has the identical result.
People have mentioned that a Windows update could have caused this and I suspect that this could be possible as this problem came up after an update and reboot.