All of a sudden, one of the two monitors stays black after start of computer

Hi all

I am at a loss where to look…

I have two Dell S716G on displayport with a Sapphire AMD 5700XT.

When running software from 2020-08-16 (Kernel 5.7.14, mesa 20.1.5) my two monitors (Dell S716G) worked fine. Meaning when starting the computer, both monitors where working and showed me a picture (not mirroring, but extending).

After the update to 2020-08-22 (kernel 5.7.15 and same mesa I think), the started to behave irratic.
When starting up, one screen would stay black, even going into standby mode.

I have to use the settings in Xfce (where both screens were usually present) and alter the non-working a little (eg. Hertz) and then its on again. Very few times only one monitor was available in settings and then I had to re-plug the DP cables.

I changed to kernel 5.8.3 hoping it might change, but it didn’t. One of the screens was still offline.

Anyone an idea where I might start looking for the issue?
Thanks a lot