Ha – sorry, stupid me – coulda have tried this before posting the above… So I did a Nvidia driver-roll-back to version 445.87 and Tataaa – it works again!!!
So indeed there seems to be something wrong with the current driver version for Nvidia GPUs – or at least in combination with the non-Steam version of RDR2 – maybe also only with the RTX2070 – that’s what I have in my rig.
Hope this helps some of you folks – especially you, “Oneword”!
@Ralf – don’t know if you are able to dive deeper into that – maybe it’s solved after the next driver release from Nvidia – for the time being I’m fine the way it works now.