Same here with me. Updated to 2021.4.0
Setup failed for spotcast: Unable to import component: cannot import name ‘KNOWN_CHROMECAST_INFO_KEY’ from ‘homeassistant.components.cast.media_player’ (/usr/src/homeassistant/homeassistant/components/cast/media_player.py)
Tried to find Spotcast in HACS to see if can reinstall but could not locate Spotcast nor it would allow me to add the repository saying it is already in the catalog.