I noticed that those two lines coincident with a certain event I can trigger:
Switching the DP-0 display off (an Eizo EV2455).
This leads to a disconnect of the DP connection and that leads (somehow) to the
quoted messages about the failed vblank.
(I'm not sure if the disconnect is actually a bug in the kernel (as it's a
DP1.2 display) or if it's my hardware/mainboard/gpu too old.)

However, this disconnect does not lead straight to the freeze.
And so far I haven't seen the bug directly after a DP disconnect, but just at
some random point.