HP 800 G3 TWR won't boot with DIMM1 and PCI-E slots plugged

[cgr3]

Hello,

I've recently purchased a second hand HP EliteDesk 800 G3 TWR PC, Quad Core i5-7500 to use as a home server. I've also purchased two HP 332T Ethernet cards based on BCM5720 chip, which I plugged into the two PCI-E 3.0 X1 slots on the motherboard. I have also installed RAM sticks into all four available slots.
The problem is this:

- If any of the Ethernet cards is plugged into the motherboard, in any PCI-E slot (tried with x1 and x16), and there is also a RAM stick plugged into DIMM1 slot, the PC won''t boot. It emits three longs beeps with red light, followed by two other beeps.

- If I unplug the RAM stick from DIMM1, then the PC will boot with both Ethernet cards plugged. In this situation, I've done some basic tests on the Ethernet cards (auto-neg, link up/down, ethtool detects them, etc) and they seem to work fine.

- If I unplug the Ethernet cards, then the PC will boot with RAM stick plugged into DIMM1. It will in fact boot with RAM sticks in all available slots, or any combination of plugged/unplugged slots, and they are detected fine in UEFI.

I've tried swapping the RAM sticks, bought new ones, plugged and unplugged the Ethernet cards as well, but no matter what I do, the PC won't boot with both Ethernet and RAM in DIMM1 plugged.
This wouldn't be such a big issue in itself, but I fear that this condition might be caused by a bigger underlying hardware problem (like a screwdriver dropped tip-first into the motherboard somewhere).
Could you tell me if the situation I'm facing is some kind of hardware limitation of this PC model, or if it is indeed an issue?

Thank you