Here an update after one month:
The "exchange" of monitors (HDMI/DP) did not help. Both monitors connected via HDMI show sync issues with 59.9 and 60 Hz. Only 30 Hz works stable. During my switching experiments, sometimes I was lucky and I got stable 60Hz. But after PC wakeup, the issue returned: Regularly black screens.
Also tried new HDMI / DVI-HDMI cables (both compatible for 4K/HDR). No effect.
October update 1 seemed to work slightly more stable (update ran flawless), after updating to october 2, the PC stopped with black screens during the update process with all three screens connected.
BTW: I once got an information after "bad" bootup with black screens: Radeon Winman (or the like) has crashed, default settings applied.
I have to use HDMI 30 Hz and it is necessary to disconnect the TV during booting.
I'm not really happy about this....