How does an HD621 handle EDID? I've seen that cause all kinds of problems with HDMI, most of which initially look like cabling issues. This post on The Poor Audiophile documents one reported solution, "The fix for HDMI woes", but I haven't tried it.

Cheers,
Simon

Roon Nucleus, Raspberry Pi + HifiBerry DIGI+ Pro, DSP3100.
Also another Raspberry Pi running Roon extensions with a Griffin PowerMate controller.
Unused: MC200.