Thanks JimH, I tried that and the issue persisted.
Although a bit later I realized that every time I opened JRiver it would pop up a Windows User Access Control (UAC) dialog asking me if I wanted to allow JRiver to make changes to my computer, a behavior reserved for unique situations, I realized that it never used to do this which reminded me that I had made some changes to test running JRiver as an administrator.... running any application as an administrator causes this dialog to appear.
It turns out that was what was causing the issue. Running JRiver as an administrator caused the drag & drop from Windows to JRiver functionality to cease working.
After changing the shortcut so that it was being ran as a normal user all functionality returned to normal.
Sorry, no JRiver bug... just a forgetful user.