What GPU are we talking about?

Your kernel is defaulting to the radeon driver instead of amdgpu, depending on which generation your GPU belongs to, you can force the kernel to use amdgpu instead. It's considered unsupported but you're still going to be much better off running the much newer amdgpu, if your hardware is still capable of using it.

This looks like a similar but different problem. https://www.linuxquestions.org/questions/slackware-14/dual-monitors-blank-screen-on-one-when-using-displayport-error-clock-recovery-failed-4175744481-print/

It seems most likely a GPU driver issue. I run Ubuntu on servers mostly, and have never used a Radeon card.

Good luck!

Check and search for dmesg errors first. Check linux-hardware.org for scans of the same machine. Look for any that are using other kernels that may have had better success. Run your own scan and upload it too so others can see.

The whole debian/Ubuntu primary use is for LTS kernel stuff. So you might be on an older kernel. All GPU stuff is kinda edgy for what the preconfigured options are set to by default. All distros have a purpose and what they are good at. The best documentation for custom kernel configurations comes from Gentoo in my opinion.

I have a problem of "unable to suspend user.slice", when using a Virtual Machine.

When I get back from sleep, I see the lockscreen, but after logging in, I have nothing displayed. This might have changed since I haven't set the computer to sleep with that VM in a while.

In my case REISUB works. REJ and switching to tty also works... but stuff doesn't work well after that until a restart