@Jappe, thanks for engaging !
Well, the splash logo, favicon title fixed itself, after a couple more server reboots, I guess.
The dev tools console in Chrome is all green, so certainly no CORS problem.
I suppose clearing Jellyfin cache and restart, and clearing dietpi cache is not enough ?
sudo rm -rf /var/cache/jellyfin
sudo systemctl restart jellyfin
Not to forget clearing Chrome browser cache !