Fyi, I was a happy camper using cubase pro 14 for a while on my windows 11 PC and when I upgraded to 15 I experienced problems with projects created before 15. VSTs would quit working if I tried to make ANY edits. Eventually I figured out the problem also existed on new version 15 projects.
I finally discovered that the version 15 upgrade loaded with WIN RT enabled. With my hardware setup I had previously disabled WIN RT to run smoothly in 14 and previous versions. I guess the upgrade overwrites that setting by default.