This isn’t really a direct answer to your question, but a bit of info just in case it’s helpful. (When I was trying to get Apprentice Alf’s tools working I also came across Epubor in my searching — and I was a bit wary, too. Given some of the comments in these MacUpdate comments — particularly the ones from 2019, wariness is probably in order.)
Anyway, for the info: your post sent me to try to remove DRM from the books I most recently purchased. I downloaded the files for USB transfer and imported them. I then converted them to EPUB format to confirm that the DRM was removed. I didn’t run into any issues, and I’m running Catalina. I have an e-ink Kindle, though, so I was able to use the serial number from that.
Are you using an e-ink Kindle serial number in the plugin? If so, is there any possibility that it’s from a Kindle that’s too old to handle the current file format?
There’s some discussion of what looks like a similar problem in this GitHub thread — ElleKayEm’s post might be helpful. There’s some additional discussion in another of Apprentice Alf’s GitHub threads.
Given that Amazon currently has their cheapest Kindle on sale for less than the normal price of a (possibly sketchy) Epubor license, acquiring one to get a current serial number may be your best bet.
And if you do happen to have an older Kindle lying around, you can get an even better deal — Amazon’s running a trade in and get 25% off promotion.
As far as I can tell, a Kindle Keyboard still works as a trade in for that purpose.