I recently had the same (or a very similar) error, but I am not sure how I solved it.
However: I was working on my laptop and when I unplugged the HDMI cable it returned to normal .(When switching from the tv screen back to the laptop screen)
I think restarting and/or opening a different level might help.
Sorry I have no real solution, but if it keeps happening post it in the bug report section of the forum.
Or alternatively on the steam workshop, hopefully someone has an answer to this problem.