Assume that you start a meeting in Skype for Business (Microsoft Lync 2013). Then, you send the meeting URL to other users. When they join the meeting, they just see a blue box instead of the videos from other users.
To fix this issue, install the November 1, 2016, update (KB3127934) for Skype for Business.
Note Lync 2013 was upgraded to Skype for Business in April 2015.