Manchineel

At the end, no solution worked for me and I changed motherboard. I switched to a Gigabyte Z370 Aorus Gaming 7, with an Intel Core i7, keeping the same GPU, case, PSU and peripherals. Needless to say, the issue is completely resolved and I haven't gotten a single 55 Q-Code yet. On the other hand, I moved the Asus Z170 Deluxe with Skylake i7 to a new case and installed 32 GB of (also) Corsair Vengeance LPX RAM, this time though DDR4@2400 MHz, for use as a server and media center with Ubuntu Linux and it's working flawlessly. Seems that the Z170D doesn't like RAM over 2666 or so.