Thanks for your reply! I already tried that - but even without hardware acceleartion it kept crashing.
I hope that the switch to this WARP-Renderer fixed it - but i only worked about half an hour with it until now, so i am not 100% convinced yet.
What makes me wonder is the kind of crash Affinity triggers - screens go black, computer restarts. Once i had a bluescreen with "Video Sheduler Internal Error".
Other programs dont crash, so i'm ruling out real hardware failures for now.
I hope it runs stable now or there will be an update (i dont feel any performance issues without the 3090 as renderer, but it feels like im not using 100% of the hardware capabilities) - so far i really like the affinity suite at first glance.