Thanks very much Scott - upgrading .NET from 4.0 to 4.5 did the trick.

Please consider offering a standalone quick support app for Windows and OS X, ideally without dependencies that may be missing.

Thanks Mhighsmith! I don't see a resolution for my issue in that thread, but thanks for the link. What I'd really like to convey is the importance of having something like TeamViewer's QuickConnect for both Windows and OS X clients; without it, SC is just very cumbersome to use with existing customers.

Here is the error:

Thanks for your reply, Mhighsmith. I have installed "Guest Session Starter Exe" to my Extensions, but am unclear on what to do next to get the EXE onto a guest machine. In the Host section, I found Extras > Download Desktop Client and downloaded it ("ScreenConnect Desktop Client.exe"), but it crashes with a "System.TypeLoadException: Could not load type..." error under Windows 7.