HP Z6 G4 with Intel Arc 380 wont boot

[IStoev]

I need some help with a used HP Z6 G4 that I recently purchased. The CPU is 2x Intel Xeon Gold 6152, for reference. It came with amd firepro w2100 GPU, which is working fine for now. I also purchased an ASRock Intel ARC A380 Challenger ITX 6GB OC GPU, to use it in a Plex VM in the Proxmox (installed on the bare metal), but as soon as I put the Arc GPU in the machine and start it, it cannot boot or even get to POST, all I get is 3 long beeps from the machine - the power LED is flashing red 3 long times and white 3 quick times (I read in the HP forums that this might be a power issue). The Arc GPU is tested on my PC and its working fine (Drivers and Firmware updated on my other system). Also I got a Dual 6 Pin Female to 8 Pin Male Adapter Power Cable for the Arc GPU as I also read somewhere that this might be the issue (I have 2x 6+2 pin from the PSU), again didnt help and still the same issue. Another observation - when I enable the Resizable BAR the behaviour with the Arc GPU is the same, but when I put the AMD Gpu back (with the resizable bar enabled) my BIOS screen is awful and almost not usable, but again maybe this is from the AMD gpu not the machine itself. Please help/advise what can I check or change to get this GPU to work.