Unable to write tmc spi 'stepper_z' register GLOBALSCALER

New Hybrid Core v4 build, has never worked (brand new).

Assembled machine, wired it, using custom DIN mount electronics enclosure.
Using Octopus EZ Max with EZ5160Pro drivers, but can repro it using an entirely independent Ocropus Pro 1.1 using TMC5610T Pro v1.0 drivers. EBB42 board.

RatOS can see the Octopus and the EZ42b, can flash them both no problem.

Any time I enter any gcode commands i.e. initial BEACON_INITIAL_CALIBRATION, or STEPPER_BUZZ STEPPER=stepper_x (using any stepper letter) I get this:

Klipper reports: SHUTDOWN

Unable to write tmc spi 'stepper_z' register GLOBALSCALER
Once the underlying issue is corrected, use the
"FIRMWARE_RESTART" command to reset the firmware, reload the
config, and restart the host software.
Printer is shutdown

I've reproduced this problem using a 100% independent Octopus 1.1 Pro board I had (new but untested).

I've swapped to a replacement EBB42 board, same issue.

I swapped the drivers themselves from x and other ports, same thing (on both cards). Also swapped the ports the motors are plugged into. Same thing.

I've swapped the Pi (same SD card though), same issue.

I've swapped in a new Beacon, same issue.

I'm running out of ideas. I 100% acknowledge this is probably something dumb I'm missing - a jumper, configuration, something. Maybe an endstop related issue? Crimps/wiring? Any suggestions?

Attached are printer.config, klipper log, picture of the Octopus EZ Max board. Happy to share anything else as needed. (NOTE: Probably alot of permutations in the klipper log with various things I've changed; sorry).

Would greatly welcome any ideas to troubleshoot this.