A few audio tracks stop playing

Weird issue:
I was working for a while on a mix and since today, 2 audio tracks stopped playing back.
I have about 10 versions of the mix (the .cpr files), and they all seem to have the same problem, since today.
I have exported mixdowns of these backup projects before the problem occurred and those exported files have the problematic tracks in them, so I’m pretty confident it’s not some setting I overlooked.
Both problematic tracks have fader automation on them, and the faders seem to follow that automation as expected.
However, when I nudge the fader slightly while playing back and hold the mouse button, the track plays. Releasing the mouse nudges the fader back into the automated position, but it immediately becomes completely silent again.
The problematic tracks don’t seem to be muted by automation or something like that.

Yesterday, everything worked fine in this project, but reopening the project today (and some previous versions) all have the same problem with the same tracks.
So I’m beginning to suspect I’ve hit a bug that somehow corrupted my project folder in some weird way.

What I tried:

  • Restart Cubase. Didn’t work.
  • Duplicate the problematic track. Didn’t work.
  • Unselect the ‘read automation’ button. Didn’t work.
  • Remove the automation on the track altogether. Didn’t work (yet! see below).
  • Re-import the audio file on the track. Didn’t work.
  • Simply just clicked everything on that channel in the mixer (disabling/enabling everything).

What did work (kind of):

  • Open the fader automation of the track, select all events and remove them.
  • This isn’t enough, I still had to nudge the fader into a slightly different position to make it play again.
  • Obviously I’ll have to redo the automation now, since I’ve removed it to make it work again.

What I did yesterday is that I installed a new plugin (Voxengo SPAN v3.1), but I doubt this has anything to do with it: Backups of the project don’t use the plugin while still having the same problem, and another project with the plugin works just fine. It somehow seems to be related to those 2 tracks in particular, and that plugin wasn’t put on those channels.