The upgrade went through. I did the following:
-- deleted unused plugins and themes
-- upgraded from moodle 3.8.6 to 3.10
-- upgraded plugins to be sure they were compatible with at least 3.9
-- ran the command line upgrade (php upgrade.php --non-interactive)
Command line upgrade from 3.8.6 (Build: 20201109) (2019111806) to 3.10+ (Build: 20210108) (2020110900.08) completed successfully.
My guess is that it was a plugin that caused the problem, and not the upgrade that happens when I take moodle out of maintenance mode.
Thanks to everyone for the help.
-- deleted unused plugins and themes
-- upgraded from moodle 3.8.6 to 3.10
-- upgraded plugins to be sure they were compatible with at least 3.9
-- ran the command line upgrade (php upgrade.php --non-interactive)
Command line upgrade from 3.8.6 (Build: 20201109) (2019111806) to 3.10+ (Build: 20210108) (2020110900.08) completed successfully.
My guess is that it was a plugin that caused the problem, and not the upgrade that happens when I take moodle out of maintenance mode.
Thanks to everyone for the help.