One more thing
I started KoReader with NM and tried to update to 2020.06-3. As expected this failed as described in
#9.
I then deleted all the files in the ota folder and started KoReader with KFMon. Again the update failed. I am given the same two options - retry with a full download or abort. I retried the full download. After a restart KOReader updated properly.
I have no idea what's going on.