@El_Artista, @devernay: Thank you both, I appreciate your replies!
I will consider the solution with switching, although it is hard to plan what to debug. I am mostly beginner, so I need to see the outputs of many nodes 
You are right, placing the OCIOColorSpace before the Writer works. The color conversion inside the Writer doesn’t seem to be working correctly - it outputs different colors. I am using ACES 1.2, so maybe there is some incompatibility (?). But I run into some quirks, so testing it is difficult.
I was even able to isolate one weird behavior and I can share a project file if anyone is interested. For a simple graph I keep switching between two values, but it acts as if there were three values. So those don’t even match with what I am seeing in the checkbox.
(I hope 600 kB gif will be ok)
It is not limited to changing a checkbox value, it “randomly” switched to black output on other occasions as well, but this is an easy way to demonstrate it.
It doesn’t do this for a freshly created project, but this is a project where I deleted other roughly 20 nodes and it happens after starting Natron again and load it.
Update: I suspected ShaderToy or Roto nodes, but after a few tests with a new project this behavior starts to appear after adding a few Read and Merge nodes. It can be “fixed” by disabling the GPU rendering in the preferences. (I am on Windows 10, with Nvidia 1070, Studio Drivers.)
Update 2: BTW I also tested setting OpenGL Contexts to 5, as stated in the troubleshooting, but it also works incorrectly. Slightly better, but still too fragile to use. So no GPU for now.