I updated my RPi3 with OSMC to buster, and found that there was an error in transmission-daemon. After rebooting, the problem persisted. I tried to launch it manually but still didn’t work. It said there was a problem with the configuration file. I checked it and found that the upgrade replaced it with a link to /etc/transmission-daemon/settings.json. But the true problem was that the file itself seems to be compressed (doing a CAT shows that it starts with TZif2, and the content is binary, not plain text as expected).
I replaced that file with a copy of my original settings.json file and now everything works fine, but clearly there is a problem in the update.