I did some googlefu and this is, as I suspected, a hardware problem
It is hitting computers with a too old intel CPU so also the APPLE MAC PRO 3,1
Which comes down to this:
you cannot use any newer AMD video cards on the Mac Pro 3,1, as the drivers require SSE4.2, which the Mac Pro 3,1 doesn't support. nVidia cards will work fine, though
But, as you can read in the link above there are workarounds like this:
And this:
https://www.reddit.com/r/macpro/comments/8xtb8u/amd_polaris_gpu_rx_series_on_mac_pro_31_with_high/
You have probably done this before:
So it seems you need this:
Or this:
https://www.intel.com/content/www/us/en/developer/articles/tool/software-development-emulator.html
Or maybe you can upgrade your CPU(s) do a model that supports SSE4.2 ?