core: media_player.play_media not working with Plex Media Server

The problem

I have been trying to play some media from my plex server to a Chromecast device or a plex client.

There is no feedback when tried for a plex client and on a Chromecast device, it just says “Sorry! Something went wrong”

What is version of Home Assistant Core has the issue?

core-2021.6.3

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant Supervised

Integration causing the issue

Plex Media Server

Link to integration documentation on our website

https://www.home-assistant.io/integrations/plex/

Example YAML snippet

service: media_player.play_media
data:
  media_content_id: 'plex://{"library_name": "Movies", "title": "300"}'
  media_content_type: movie
target:
  entity_id: media_player.family_room_tv

-----------------------

service: media_player.play_media
data:
  media_content_id: '{"library_name": "Movies", "title": "300"}'
  media_content_type: movie
target:
  entity_id: media_player.plex_plex_for_mac_local

Anything in the logs that might be useful for us?

No response

Additional information

No response