Well, I did that. Don't have virus checkers running, though, and I never have other programs running during installs.
Result: At least error #91 is still there when I want to alter the altitudes of waypoints (it's even there if I do nothing and just close the window). Can't tell about the frequency bug yet, have to rerpeat the flight (with a newly compiled flight plan).
One other thing I have noticed with the old install: on approach to LOWW, I got at least three or four runway changes before ATC decided where to put me down, in extremely fast succession. Is this normal?