Steps to reproduce:
1. You should have Stream Security enabled. At the time of this writing, the code is not officially merged in 1.6.x (Pull request #628)
2. Make sure you have distributed a video via download.
3. Try to watch that video in the Engage Player. Make sure to add the URL parameter "&vmode=progressive" to force progressive download instead of streaming (in case both options are available).
The player does not load. A "Stream could not be found" error is displayed
The video should play as long as the "stream security" signature is correct