Why can't I exit Dreamweaver CC?

Question

I've just installed Dreamweaver CC on a test machine, and while it launches OK (except for a couple of irritating proxy authentication dialogs) it won't exit.  That is, selecting File->Exit or pressing the close button in the corner of the window does nothing.
This is a packaged install (using Creative Cloud Packager) created earlier today.
Any ideas?

Answer 1

Additional: it isn't due to use of the Creative Cloud Packager.  I've just done a normal trial installation on a different test machine and the same problem occurs.
Also, Dreamweaver CC will exit successfully (with no need for a forced exit) if the user logs off.  So it isn't incapable of exiting cleanly, it just doesn't seem to recognize what I'm asking it to do.
Additional #3: I've just reproduced the problem on a fresh standard install of Windows 7 x64 SP1, with no configuration changes other than setting the proxy server.  So it really doesn't seem to be an issue with our environment.
Additional #4: it's looking as if the problem only occurs on machines which access the internet via a proxy server.  I'm still investigating.
Additional #5: to further complicate troubleshooting, if the machine is ever given direct access to the internet the problem goes away and won't come back.  (Looks like this is going to be a doozy.)