Symptoms
A user can't join a Skype meeting after you run the following cmdlet in PowerShell in a Microsoft Skype for Business Server 2015 environment.
Reset-CsPoolRegistrarState -ResetType FullReset
Resolution
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, core components.
