Symptoms
Your end-users report that they can't join a Skype for Business meeting, especially when they join the meeting by using Microsoft Business Room System or Surface Hub devices.
Cause
This issue occurs because all subsequent sign-in processes fail for these devices after a rare race condition occurs in Microsoft Skype for Business Server 2015 Front End service.
Resolution
To fix this issue, install the December 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015, Front End server and Edge server.
Workaround
To work around this issue, restart the Skype for Business Server 2015 Front End service to have the issue lessened.
