Help needed! Nvflash in APX mode => transport endpoint shutdown

I’m in the process of trying to use Tegra124 SOC’s in a series of bricked Tegra tablets. (Purchased “as-is” without backups or anything). I have resoldered them with brand new battery packs and would like to use them in an educational art project.

The first device I’m testing only boots in APX mode:

[170382.764903] usb 1-3: Product: APX
[170382.764909] usb 1-3: Manufacturer: NVIDIA Corp.

Trying to run “Nvflash --create” gives me the following:

Nvflash 4.13.0000 started
chip uid from BR is: 0x64001001740dc1831c00000014028040
usb read error (108): Cannot send after transport endpoint shutdown

Please Help
I would love to experiment with this board, is there a way to revive these tablets?
The behaviour seems consistent with the description of a PKC or SBK locked device.