Symptoms
You cannot use a third-party browser to sign in Skype for Business Web Scheduler in a Microsoft Skype for Business Server 2015 environment.
Cause
This issue occurs because the origin header isn't handled correctly.
Resolution
To fix this issue, install theĀ DecemberĀ 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015, Web Components Server.