I had this same issue in Cubase 10 and 10.5. Now I get it in Nuendo 10.2.10.
Kontakt 5 works great, no problems. However, Kontakt 6 will work for a little while but then loading any library causes it to crash Nuendo. I had stopped trying to get K6 to work smoothly but since I’ve been using Nuendo since NAMM I thought I’d try again. Nope, same crashes.