I think something's wrong "out there in the cloud."  

I just tried the same thing with Add-On Updater Version 21.09, and got the same error.  After removing that version of the add-on, I cannot download the latest version, which is 22.01.  I am getting a "502 Bad Gateway" error when attempting to fetch the latest Add-On Updater from:  https://addons.nvda-project.org/files/get.php?file=nvda3208 

I'm going to send a quick email message to the folks at NVAccess.  I'd just wait a while and try later.
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Brian - Windows 10, 64-Bit, Version 21H2, Build 19044  

The instinctive need to be the member of a closely-knit group fighting for common ideals may grow so strong that it becomes inessential what these ideals are.

       ~ Konrad Lorenz (1903-1989)