Okay, "hiccup" might be an understatement. Something beginning with "cluster" might be more appropriate.
I tried to upgrade my MythTV backend box from Linux Mint 19.3 to Mint 20, using the Mint upgrade tool. Even on a fairly fast machine with a fast Internet connection and not much installed on it (MythTV plus the applications that come with Mint), this takes hours. A seemingly endless series of commands scroll in a terminal, and I don't dare walk away for too long, less the process stop waiting for some input from me (it periodically needs my password) or due to a glitch.
Speaking of glitches, I noticed that the scrolling stopped and the process seemed to freeze just after a couple of lines about installing symlinks for MySQL and MariaDB, two database programs. MariaDB, which I've never had installed before, is apparently a fork of MySQL. MythTV uses MySQL as its database manager. Before upgrading, I had killed the MythTV back end, but I noticed that the MySQL server was still running. On a hunch, I opened a separate terminal and shut down the MySQL server. Sure enough, the upgrade process resumed, with a message about a cancelled job or something, which I think referred to MariaDB. Whether this contributed to the unfolding disaster I do not know.
After a reboot, the good news was that everything that should start did start, and the frontend was able to see and play the recorded TV shows. The bad news was that (a) the backend got very busy doing a lot of (alleged) transcoding and scanning for commercials that should not have been necessary (having already been done on all recorded shows) and (b) I could not shut down, because the backend thought it was in a "shutdown/wakeup period", meaning (I think) that it thought it needed to start recording soon -- even though the next scheduled recording was not for a couple of days, and the front end was showing the correct date and time for the next recording. So I think the switch from MySQL to MariaDB somehow screwed up something in the database.
From there, things got worse. I had backed up the database, so I tried to restore the backup (using a MythTV script for just that purpose). The script failed because the database already contained data. Following suggestions online, I dropped the relevant table from the database and tried to run an SQL script (mc.sql) to restore a blank version of the table. No joy -- I needed the root MySQL password, and no password I tried would work. There is allegedly a way to reset the root password in a MySQL database, but that didn't work either, and in fact trying to shut the server down using "sudo service mysql stop" did not work (!). The only way to get rid of the service was to use "sudo pkill mysqld".
Fortunately, timeshift was able to restore the system to its pre-upgrade state (with a little help from a backup of the MythTV database and recordings folder). For reasons I do not understand (which describes pretty much everything discussed here), restoring the database backup did not cause MythTV to remember this week's schedule of recordings, but as soon as I reentered one (using MythWeb) it remembered the rest. And I thought my memory was acting a bit quirky ...