So I have some news. I had the same problems in IRAY mode in 4.21 as with 4.22. The program crashed all the time. I found a solution in another forum that at least works under 4.21, it hasn't crashed for a few hours now. I still have to test 4.22.
My CPU and my graphics card were activated under the advanced render settings in photorealistic mode and under interactive mode. I deactivated CPU in both windows and voila it runs without a problem.
Maybe someone else can test this under 4.22. I hope this might help a few people.