Hello, guys,
looks like I've found the rootcause. I have 2x16Gb Kingston HyperX Predator 3000MHz RAM modules, and seems like one of the modules is defective. Memtested them separately in different slots: first module - no errors, second one - a lot of them. And the system boots and operates just fine when only the first (good) module is installed (had no (A7) ME FW Downgrade error). With the second module it takes considerably longer time to boot up and there is the error.
I'll return the modules to the retailer tomorrow and will ask for replacement.
Thank you for your help!