Next confirmation: ECC works on Gigabyte's X399 DESIGNARE EX board just fine.
I tested with 4x 16GB 2Rx8 modules (CT16G4WFD824A) and 8x 8GB HPE 2Rx8 unbuffered ECC modules.
In fact I was able to boot the machine with 4x 16GB + 4x 8GB resulting in 96GB ECC RAM :)
Linux 4.14 kernel boot messages...