Path Tracing strange - what's wrong?

Hello,
several times I tried Path tracing, it’s always looking wrong.
I followed all the official instructions.

directX12
raytracing
path tracing enabled…
tested over hours all settings with all ray tracing option (max rays on/off)

scene looks always black in refractions.
Last test in current UE 4.25

I have two gtx 1080ti with latest studio driver. no idea what’s wrong…
Thanks