UE5.3 | Viewport Bug?

Hi there
I seem to face a viewport problem:
If i’m in perspective mode, everything works - as soon as I change to another viewmode, such as top, front, back… etc. nothing is shon. no matter if as standalone viewport or split-screen as shown in the pic below (sometimes, UE even crashes when I change the viewmode from perspective to another)- any clue how to fix this? Thank you