You need to check carefully which pci-e riser port use to have more resources available for the card.
Check the intel manual for the board page 29: https://www.intel.com/content/dam/support/us/en/documents/motherboards/server/sb/s2600gzgl_tps_r2_4.pdf
Try to plug the card with different pci-e config. Would say on the riser slot 2 that is connected on the 2nd cpu with more resources available i suppose.
Tried it on riser slot 2 (was only trying slot 1 hence the error) and the
system won't even boot. I even disabled 3 out of 4 on-board Nics and
removed other devices plugged to the PCI interfaces and other cards (10G).
System has been updated to latest BIOS (02.06.006) as well.
Is this motherboard supported even? Or do we need to get the
officially-supported systems from supermicro?
Well sorry to heard that.
Last thing it come in mind is the kind of riser you are using.
Maybe you need to use the single pci-e 16x riser without any other derivation.
There are different kind of risers, try with the other ones on both riser slots.
Also this machine is powered by one or 2 cpu? Maybe there is support only with two cpu, where with only one, the resources over the pci-e are already bloated with all the other stuff.
As last thing try to mail the AMD support about, as also the Intel one.
I'm using the 2U riser variant with 1 x 16x pci-e on the top and 1 x 8x on
the bottom. Sorry, don't have access to the singe 16x riser card so can't
test it with that. Also, this is a 2CPU machine.
I had a look at the supermicro boards and they have 16x PCI-e ports
on-board. Would this be the main issue? I can't see in the intel docs the
slot type (16 or 8x), only the number of lanes.
So the 2 risers slots are the PCI-e adapter ports, if i got it well there is no direct simple PCI-e 16x port like the one you will found on a normal desktop board or on others server boards.
Be sure to have both risers plugged into the riser slot even if you are not using anything on the second one.
Then nothing more come into my mind, one would shut down any other unused integrated hardware that use PCI-e resources as you did.
You can also check these link i found:
S2600GZ + nVidia Quadro M2000 |Intel Communities => Revert to previous bios + bios config.
FAQ Entry | Online Support | Support - Super Micro Computer, Inc. => PCI-e bios config.
0146-PCI Out of Resource Error for Intel® Server Board Product... => PCI-e bios config.