C4D R20 and Win10 (current) and I found a bug and did a lot of testing to pinpoint it (yay):
Enabling Cryptomatte "Instance" crashes 2021.1.1 and C4D while it is trying to save the EXR(octane):

I could provide a scene but it's really just a camera, a cube and a daylight object. No materials.
In 2021.1.R7 it works fine.