When a user tries to join a Skype meeting with Skype for Business on Mac by clicking the meeting link in Outlook, the Skype meeting can't be joined.
When Skype for Business on Mac tries to join a meeting, the client passes the meeting URL to Unified Communications Web API (UCWA). In some cases, there can be additional parameters in the URL that are not recognized by Skype for Business Server. Therefore, the meeting can't be joined.
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, Web Components Server.