Symptoms
Considering one of your Microsoft Lync Server 2013 Front End servers stops responding, it might be back to usual about 3 to 5 days after you restart the RtcSrv.exe (Skype for Business Server Front-End) service in the problematic server.
Resolution
To fix this issue, install the July 2018 cumulative update 5.0.8308.1001 for Lync Server 2013, Front End Server and Edge Server.
