I can’t wait to find out what the problem was as well.
Sounds like a phase canceling issue and the mentions of stereo expanders, fix it at the mix buss and pyscho acoustic type plugins has me suspicious. I’ll be surprised if its the operating system.
One person mentioned not going to 64 bit above because they have so many 32 bit plugins. That confuses me as I run 64 bit Cubase and still use my old 32 plugs as well so confused me and wondering what that meant.