Yes, that does the trick it appears. I wonder though, because I incidentally ran into this behavior for a long time, also before Cryptomatte was added to OctaneRender.

Thank you for reporting, Funk!

It is a real workflow killer in render-intensive scenes to not being able to use the region rendering. I have to wait for the whole frame to reach 60 samples and then can select a region. However when I want to view another region it gets stuck with CUDA errors often.