no-boot, no-video when adding an AMD Instinct MI50 in my 7820

I have a Precision 7820 workstation with 2 Xeon 4110 CPUs and an AMD WX3110 video card.   It's working well with Proxmox (Debian Linux).

I bought a brand new AMD Instinct MI50.  As soon as I install it in the workstation, it refuses to boot, and displays nothing, not even de Dell logo, on the Displayport output of the WX3110.

I had disabled Secure Boot to install the OS, so this is not it.   

I tried these:

• the WX3100 was originally in the 1st PCIe slot (the one closest to the CPU), and the mi50 in slot 2. No video.

• I moved the WX3100 in slot 2 and the mi50 in slot 4 (following the engineering diagram from this thread in the Dell forums . Still no boot.

• I tried setting "legacy option ROMs" (AKA CSM), no effect.

Nothing works until I remove the card.    I can't try this solution, because the workstation never makes it to try booting so it never goes through the "missed boots".

Anybody has an idea of what is going wrong?