For me, also, sudo do-release-upgrade -c
reported no new release.
So, next I tried upgrading 19.04 to 19.10 using
sudo do-release-upgrade -d
(instead of -c)
The good news is this resulted in an upgrade to 19.10 after a couple of hours with brief pauses for permission to use distribution versions of a couple of files instead of my customized versions. However, the bad news is that I had no GUI login prompt after the upgrade. I could SSH in remotely but could not login locally. I tried fixing the problem manually and it didn't help.
Next, I tried this again on another computer. The end result was the same as the fate of the first computer.
After that, I restored the two computers from the backups I made just before attempting the upgrades. Both computers are back on 19.04 and are working properly. I will try again in a few weeks.