Since a20.4 custom prefabs were stored in the save folder. Nuke everything in the Save folder. When we see those errors, it is usually because you have modded prefabs there.
As for the server admin file. A lot has changed since a20. Backup your old file, and delete it. Then let the game make a new one. Then add your admins in via the console. You can then copy the blacklist and custom command permission sections over to the new file if needed.