Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 660 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.86
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-24 14:22, hash: rB507ffee6e1f4
Short description of error
I exported a multi-layer exr with cryptomattes for objects and materials multiple times, but couldn't get the Cryptomatte node in Nuke to recognize any cryptomatte information. After searching for a solution, I found this (https://blender.stackexchange.com/questions/129538/how-can-i-render-cryptomatte-pass-for-nuke) in which the #1 solution is to change the View Layer's name to "Scene". So I tried that, having exhausted all other alternatives, and by jove, it worked perfectly. So what this means is, you can currently only export cryptomattes (that will work in Nuke) from one single View Layer, and that View Layer had better be named "Scene" or you're screwed. Not sure if that's an issue on the Blender side, or if the developers of the Cryptomatte plugin need to work on that, but I figured I should bring it up.
Exact steps for others to reproduce the error
Render any scene with Cryptomatte and the default view layer name, save as a multi-layer exr with ZIP compression, import into Nuke, connect Cryptomatte node, and see that the dropdown doesn't see any cryptomatte information. Try again with view layer named "Scene" and watch the magic.