Hello there,
I think my RX 550 stopped working because of this. I swapped the card in to a Bulldozer based system and swapped it back a few days later. My Optiplex refuses to boot with that card inserted (solid amber light, led 2 3 and 4 flashing) The card itself works fine, just in this specific PC it doesn't work anymore. (Yes other older cards work fine in the Optiplex)
Would be anyone so kind and is able to remove this UEFI stuff from my BIOS?
Thanks in advance!

@Aphelion - Did it work before in the optiplex, or no? If it worked before in there, but not now, then something you changed in the BIOS is the reason it's not working anymore.
If it never worked in the optiplex, you need to correct your comment >> The card itself works fine, just in this specific PC it doesn't work anymore << Remove
Here is GOP removed - https://ufile.io/l9k9623m

@Lost_N_BIOS - That was realy fast, thank you very much! I will try the BIOS when I'm home.
No, the card did actually work before I used it in a different system.
I also didn't changed anything in the BIOS. I started messing arround with the settings when the error occurred. Yesterday I tried other older motherboards, an ASUS Ion Motherboard with a Intel Atom D330 and a realy old ASUS Athlon 64 x2 bord and there I also cannot get any video output, but in two different AMD Bulldozer PCs and an Intel z170 based system, it works like a new card straight out of the box.
I literally changed everything in my Optiplex besides the Motherboard to find the issue, but nope no luck there.
The modified VBIOS is my only idea left why the RX550 doesn't like the PC anymore. I don't want to build a new HTPC as long as the MSI z170 8 thread lock isn't gone. Then I can swap my 6700k to a 9900k and use my old 6700k for a new machine.
So I guess my grandfather is right. "Never change a running system."
I will post an update when I tried the new VBIOS.

@Aphelion - Thanks, and you're welcome!

Yes, but you said "Work in this PC anymore", as in, at some time in the past, it DID work in this optiplex before. Which, sounds like it has not, maybe, so you should not say that it doesn't work in this PC anymore (if it never did)
Or, are you saying, and please clarify, this graphics card DID work in this Optiplex system before? If yes, then something you changed in the BIOS would be the only reason it is not working now. Booting the card in another system would not affect it at all, or how it works on any system
Did you wait and let it boot to windows? If you change the boot mode (UEFI/Legacy) or OS Type (Win7-10, or other), then it will work differently now than before, or if you didn't load optimal defaults when you put it back in there (and you did before), then settings may not be the same etc.

With the vBIOS above, you cannot boot w/ secure boot enabled, or in UEFI mode, that has Legacy vBIOS ONLY now. It should also then boot in your old AMD systems you mentioned, but sometimes vBIOS is not the only reason for older incompatibility, so it still may not work on those.

@Lost_N_BIOS Yes the exact same RX 550 did work in this exact same Optiplex for about 11 months. I never changed anything in the BIOS. The only thing I did in 2017 when I bought this PC was to load optimal defaults.
I took the GPU out. There were no other things I did.
After I installed the card back in the PCI-E Slot, it stopped working. I installed a different older AMD card and clicked the "load optimal defaults" button to make sure that the settings are still the default settings and nothing had changed.
The Dell Bios has no options for UEFI/Legacy Boot, Secure Boot or OS Type. (I'm on Win 7 if that matters)
And also, I let the PC run for about 20 - 30 minutes and still no successful boot.

The VBios flashed without errors in atiwinflash and the card still works in my other PCs, but unfortunately it still produces the error I mentioned in an earlier post. Which is a shame because that was the last idea i had.

@Aphelion - Thanks for clarifying! So, with the RX 550, once you put it back in the optiplex, you should load optimized defaults again too then.
Did you do that during your troubleshooting? Sounds like you did for a different card above, but not this one, this time?
Sorry to hear it's still not working! Please try again, load optimal defaults and reboot, then if no luck, flash back the original vBIOS, and then again load optimal defaults.
If still nothing, that is VERY odd! Clean the gold contacts and blow out the slot to see if that helps. When you try other cards that do work here, do they run at the full x16 width?
You can check with GPU-z, you may need to install drivers each time, and load the graphics test "?| beside width, to run 3D test to make it ramp up to full width.

@Lost_N_BIOS I can't load optimal settings with the RX550, because the Optiplex won't start anymore with the card inserted. I tried it with the CMOS Jumper but no luck there.
But your suggestion with GPU-Z and the PCI-E bandwith could be right. My test card (r7 240 OEM) and my RX 550 are both electrically wired with PCI-E 3.0 x8.
When I stress the r7 with the included 3D Test, it should tell me "PCIe x8 3.0 @ x8 2.0" (because the Optiplex doesn't support 3.0) but it stucks on PCIe x8 3.0 @ x8 1.1 which is defenitly wrong and Windows feels kinda sluggish.
I can remember that I had this problem with a PC in the past, but I really don't know anymore how I fixed this error. I know for sure that it's not the software. I deinstall my drivers before I swap GPUs with DDU and it never failed me.
But that still doesn't explain why my RX 550 can't boot and my R7 can.