Question No post with GPU

[YourObserver]

Hello everyone,

I didn't use my PC (Hardware below) for quite some time and now have issues with black screens. It has worked flawless before that.
When I install the GPU the PC does not post at all. I cannot go to bios and hence not enter Windows desktop.
When I remove the GPU and use the APU everything works just fine. I tried the following things so far:

1. Remove GPU, use APU to update MB Bios to the latest version; worked succesfully, no different result
2. Remove GPU, use APU in safemode to uninstall all previous drivers using DDU
3. Install GPU and launch PC connected to the APU, blackscreen again.

Some additional information:
The GPU is connected via 6 and 8 pin to psu. It is connected correctly as indication lights as well as the RGB light up.
There seems to be no difference when being connected to APU or GPU, if GPU is installed.

Windows 10 related issues could be resolved by using Inplace-Upgrade.

Thanks for any advice to solve this issue. If any more details are needed please feel free to ask. As English is not my first language I might have some difficulty in understanding any technial terms so please bear with me if I need to ask questions again.
Best regards

Hardware: MB: MSI B450 Tomahawk (BIOS: 7C02v1E ), CPU: Ryzen 2200G, GPU: Asus Dual RX5700, PSU: be quiet! PurePower 11 600W

 

[NightHawkRMX]

Do you have another system to test the GPU in?

 

[YourObserver]

Unfortunately I do not have a system on hand. I could test the GPU on the weekend, but only in a prebuild Dell. I don't know if the PSU if powerful enough though, as it is my work-PC.

 

[YourObserver]

In the meantime the problems got weirder. After booting to windows in installing chipset drivers I do not get any sound output through the audio jacks. The only audio I get is through HDMI output and the monitor. Could that hint towards a faulty MB?
My next steps will be reinstalling windows and then probably swap out the MB in some time. Thanks for any contributions