Motherboard. Chipset includes memory controller...
Afaik there is no RAID controller in KRC2. Doesn't make sense, there is only one harddisc in the controller, connected directly to motherboard.
ECC error normally comes from main memory (RAM), Didn't know that they had ECC memory, Thought that they have normal RAM, without ECC. May be the BIOS settings are wrong,
Yup ... ECC check is related to RAM, not storage memory.
So things that could lead to failed ECC check are faulty RAM, memory controller, CPU, motherboard, or as hermann already pointed out, simply wrong timing.
It costs nothing to try loading defaults, or intentionally select slower memory access.
External interference would be very unlikely since both motherboard and PSU are shielded, in a sense acting as Faraday's cage
