Symptoms
The RTCSrv.exe process crashes on all the Front End servers in all the pools that you have in your Microsoft Skype for Business Server 2015 environment. Additionally, the following error message is recorded in Event Viewer:
WRTCES.dll!HttpIncomingRequest::CheckIfCertPresentAndValid
Resolution
To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for Skype for Business Server 2015, Front End Server and Edge Server.