If it works with the others on their default theme, then I guess it must be something on my end.
This is the code that I am using in ui-lovelace:
- type: custom:mini-media-player
entity: media_player.bad
icon: 'mdi:cast'
hide_power: true
show_source: true
title: Sonos Badezimmer
more_info: false
Anything that may seem wrong here?