[bbsc]

well, S3 is suspend to RAM - a flakey module could cause issues here 

Thanks, I'll test it carefully. The idea deservs consideration.
But I don't believe it is faulty because it withstood  the complete Windows 10 installation, Office 2019 installation and full update procedure without any problem.

There's a good method to check if this is a firmware problem: I can update my second Zako to the latest UEFI firmware and see what happens.
The problem is that I badly need S3 mode on the second machine and don't want to take this risk.

Well, seems it's time to look where is my old good SPI programmer....