From the HD621 manual:

Meridian Comms BNC and DIN connectors – these enable the processor to be integrated
with a Meridian audio/video system and controlled by it, for example, switching the HDMI input when the appropriate selection is made via the system controller.

Connect an HDMI source to each of the inputs you wish to use. You will see that the connectors
are named according to the standard convention adopted by Meridian equipment. If you connect
your sources as suggested by the labels, you will not need to change them in the Meridian
Configuration application.

This all implies to me that the HD621 is not usually used as controller, and also that this should basically just work as long as nothing has been changed from standard, and nothing is broken…

The manual does also state:

Note: the HD621 can act as a Controller. However you will only use it as such in a non-
Meridian system. If you select the HD621 as the controller in a Meridian system, the available
inputs on a Meridian processor you add subsequently will be limited to those defined for the
HD621.

If it were me at this stage I would:

- swap out the BNC cable
- reset/retype DSPs and HD621 to standard settings

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502