Xibo update 1.8.2 to 2.3.12: no daypart appears, it is empty

I suspect you have deleted the first super admin level user in the system and now the dayparts are owned by a user that no longer exists.

You’ll need to run some SQL on the database to resolve that.

UPDATE `daypart` SET userId = (SELECT userId FROM `user` WHERE usertypeid = 1 LIMIT 1) WHERE userId NOT IN (SELECT userId FROM `user`);
UPDATE `display` SET userId = (SELECT userId FROM `user` WHERE usertypeid = 1 LIMIT 1) WHERE userId NOT IN (SELECT userId FROM `user`);

If you need guidance on running SQL commands, there’s a document here which explains for a Docker-based installation: