To recap.
You changed the CPU and motherboard? Are you saying you have used two different motherboards, and two different CPUs and you have the same issue with only one set of memory slots working?
Please remove your CPU and inspect the socket for misaligned pins. Then carefully fit the CPU and the cooler once more.
Now follow the guide in my sig and run Memtest86+ Please do follow the steps in the guide.
What I could suggest is that your use one stick of memory and select XMP profile as your mode of OC F10 and enter to save and boot to OS with one stick. Open CPUz and look at the memory tab, if it shows 1200MHz then you are set up correctly. Double data rate, so 1200x 2 for your actual memory frequency.
Now power down and add the other stick.
Power on and go into OS you should be good to go.
If you get the 55 error. Swap the stick of memory to another slot and try again.
But please do use the Memtest86+ guide first.
To help us help you - please provide as much information about your system and the problem as possible.
