I set up a new Gitea server where I migrated all user accounts from an old Giea server to the new one.
I migrated those user accounts by simply recreating accounts with the same names there.
The strange things happened that some repositories would be shown in the Drone web UI for some accounts but not others. Some accounts would show no repositories at all. I am sure the URL/IP settings in my docker-compose.yaml were changed properly.
It turned out that I have to delete the database files stored at /var/lib/drone. After that and restarted the drone server, everything seemed to work normally.
That is probably a bug but anyway I posted this just for someone else encountering the same problem.