Mr J D Thomas - how do I buy you a beer or send you cookies or something!?! 

THIS is the fix.  No need to downgrade Parallels or uninstall recent Windows updates - changing the Emulation settings on QBD did it. I can't tell you how grateful I am.  I fought with this one all weekend and there was no info and Intuit support wasn't even available.  Installing QBD on a PC laptop and moving the company files over got me going, but this totally fixed it on Parallels VM where I actually want to use it.   I had the same problem as you; couldn't open company file, couldn't open Sample file.  Reinstalled QBD. Did every single thing suggested in File Doctor. Nada.  Monday Intuit support suggest the company file needed repair by them, but then were a bit stumped when I told them I couldn't open a sample file either.  They need this fix posted everywhere!

Again, many thanks for contributing to this community with your knowledge.