DVR Recordings work, but Live doesn't

I’ve been using Plex DVR for over a month now, with the following setup:
Digital Devices 4-tuner card + TVHeadend + Antennas proxy + XMLTV.xml webgrabplus EPG feed (Gracenote sucks for Belgian TV)

My Program guide is filled up nicely and I can plan recordings. The shows are recorded flawlessly.

BUT:

  1. Watching the recorded show without transcoding gives me all black video + sound on all devices (Apple TV, Web, iOS). Transcoding fixes this
  2. Most importantly: Watching LIVE never works. I always get an error as displayed below.

Any ideas? I know the setup is not officially supported, but since recordings do work … I was hoping there’s a simple fix?

! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] DEBUG - DVR:Schedule: Attempting to schedule the item ‘Live TV - Session’.
! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] DEBUG - DVR:Schedule: Trying airing at 2018-03-26 14:29:19 on channel Canvas%20HD.
! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] DEBUG - DVR:Grabber: Operation for Live TV - Session completed with status error (The device does not tune the required channel)
! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] DEBUG - DVR:Grabber: Cleaning up rolling grab for channel Canvas%20HD.
! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] WARN - DVR:Schedule: Couldn’t schedule recording (tuned channel: 0)
! Mar 26, 2018 14:29:19.837 [0x7fe27c3fe700] DEBUG - DVR:Schedule: Attempting to unschedule lower priority grabs, we have 0 grabs to look at.