Users can't join a meeting through a meeting link in Skype for Business Server 2015 (KB5005519)

Symptoms

In a Microsoft Skype for Business Server 2015 environment, users intermittently can't join a meeting through the meeting link. However, they can join the meeting by using the Microsoft Skype for Business client or Outlook. Additionally, users may receive the following error message:

Resolution

To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.