Trent,I've done some more digging, and noticed that "C:\ProgramData\MicroFour\ConnectionData" has, amongst others, permissions that allow Users on my PC "read and execute" as well as "special" permissions ("Create file / write data", "Create folders / append data", "Write attributes", and "Write extended attributes") both inherited. However "AppKeys.dat" and "Connections.dat" don't have the special permissions.
Then I tried deleting the files and seeing what happened when they were recreated. First I ran one of our programs as administrator, the files were created with no special permissions for users, and the program ran fine. Subsequently running another of our programs as me, I got the exception we've been seeing. Then I tried again deleting the files and running a program as me. The files were created with permissions the same for users as before (but I also got extra permissions as owner), and the program ran fine. Subsequently running another program also worked fine. I don't dare try running as another user that isn't administrator. I haven't consciously changed any permissions settings, and there is no group policy running within our network that I know about (and I'm allegedly our expert on group policy, i.e. I've read a few chapters of a book).
Moral of the story: I'm confused and think making the location of the files configurable is a good idea.
Peter