I run a 50ft HDMI cable between the HD621 and a Sony ruby projector, and have never had a problem. The cable has serious shielding.

I'm wondering whether swapping cables, just as an experiment, might be worth a try.

Living room: 818v3, Parasound A23+, Maggie 1.6
A/V room: 861v8, DSP7200SE L/R/C, 4x DSP320 surrounds, ID41 + Roon, UHD722, Prime+PS