I can confirm Peter's experiences, but will add that his solution of using Quicktime as an external player only works if you also have iTunes installed on the same PC, and that PC is "authorised" via iTunes to play downloaded content.
This is not a very good solution, as the playback via Quicktime (or iTunes instead) doesn't use MC's video and audio processing, or controls. So the movie I watched over-drove my speakers on one occasion, with a loud pop, and I couldn't use my remote control to control playback. Plus it runs in a separate window, which on my HTPC for some reason doesn't size correctly until I maximize it. It is a pity that the included Quicktime Engine doesn't work. Even with iTunes installed and authorised, with or without file association set to play with MC, only a flat grey screen is shown with no audio. I couldn't find any information to correct that problem. Most information was years old, and referred to a time when MC relied on Quicktime for playback, instead of using Red October.
System:
Windows 7 Ultimate 64bit SP1
JRiver Media Center 19.0.120
iTunes 11.1.5.5
Quicktime 7.7.5 (1680.95.13)
Any chance of a future fix to this issue? I know, I know, it is Apple, and they hold DRM close to their hearts and wallets, so the answer is probably no.