Thank you for posting, I never thought to post this fix. I ran into the same issue with our environment, we have multiple modules.
I did not to a clean install meaning I did not remove the old, or change servers. Like you I deleted the registry settings. However I did them all except for agents. IT took me a while but then the upgrade went smooth.
We have several environments, I did not have to do this on any main pollers or the EOC, but new APEs on Windows 2019 is where I ran into this issue, although I suspect it could have cropped up anywhere.
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