I have gone back to a UD3H after benching on a GD65 for a while and am having the same issue. Binned all my ram on UD5H, none reached 2600 9-12-9 on air so I tossed them to a side, got a new kit and tried in the GD65 and 2600 9-12-9 1.8v 32m passing no issues, back to the UD3H and "51" error with 2600 multi or 2400 multi and bclk up to 2600. Tried up to 1.9v and no difference.
I tried adjusting the reference ram voltages to .89 - .895 but that hasn't helped, relaxed subtimings and still a no go. I'll have to setup my GD65, check subtimings and apply them to UD3H. Hopefully then I can boot at 2600.