I'd bet what happened to you is the same thing that happened to me. I've got a very similar motherboard and I kept getting the same error code 55. This is what I built:
GA-Z170X-Gaming 5 (Rev 1.0)
i5-6600k
2x8gb Crucial Ballistix Sport DDR4 2400
EVGA 1050W PSU
I was able to test those RAM modules in a server and they worked, so it was not an issue with the modules. My guess was on the 2133mhz socket and the 2400mhz modules (which are in the compatible RAM list, but anyway).
And I was right. I got another memory kit 2x4gb G.Skill RipJaws IV DDR4 2133
I specifically wanted to try with the smallest & slowest RAM to maximize compatibility, so I went for those.
I put a single 4gb G.Skill RipJaws IV DDR4 2133, and BINGO! it worked. I got into the Bios setup, Bios version was F2.
I tried 2x4gb G.Skill RipJaws IV DDR4 2133, it worked.
I changed the RAM frequency multiplier from "Auto" to "21.33" which fix the memory speed to 2133mhz, then removed the Gs.Skill modules and tried with 8gb Crucial Ballistix Sport DDR4 2400, IT WORKED! Also two of those worked together.
Went back to the bios, the bios was reading those modules as 2400mhz (and it even recognized that they had an XMP profile) but forced to run at 2133mhz by my previous configuration. Tried to switch back to auto > Code 55 AGAIN! (with the XMP profile switched off & with the XMP profile switched on).
Back to G.Skill, back to fixed 2133 speed, back to the Crucials, it worked again. So far as long as I didn't reset the CMOS, it was working.
I tried running all four modules at the same time, it worked. In order to do that, at least in "Auto" speed setting, the G.Skill 2133 had to be in DIMM1 (red), then another G.Skill 2133 in the other red socket and the two Crucial 2400 in the black sockets.
Then I updated the bios to the latest version F4. Nothing changed, all tests produced the same results as before.
Right now I'm running all four modules with a total of 24GB and they are running at 2133mhz. The only way I managed to run the Crucial 2400 modules is by forcing the bios to run the memory at 2133, either by fixing the memory speed or by putting the 2133 module in the first slot.
However, in order to get this system working, I see no way of doing it without buying 2133mhz DDR4 as I did or at least borrowing from someone else. They problem is that if you don't keep that 2133mhz ram, as soon as you clear CMOS, your system won't start anymore (because it will go back to the default "auto" speed for RAM").
I'll contact tech support to make them aware of this issue so they can release a new Bios version to fix this.