I ran into the same problem today too and am glad for this thread here. My versions are today:
PT8 version 8.0.100.99
FW 9.0.11.0
For the record: No project on this PT installation is at all linked to anything FieldWorks (we had trouble in the past and never want that again).
Just for fun, I tried to run the commands from a Power Shell and it did not work at first, but when I run the Power Shell as administrator (Win 10 pro) it works. The third line then reads like this:
.\Paratext.exe
I made a batch file and it also needs administrator level to run, no problem, just a note to myself.
It feels to me, that PT is far too keen to link-up with FieldWorks, especially when the users have never used that feature at all. So this problem should not even exist, whatever versions are installed, for users who are not linking.
And the error message that comes up is not helpful at all as this is all about versions not liking each other and not about Fieldworks not working properly.