If running Lync Server 2010, Lync Server 2013 or Skype for Business Server 2015, and started noticing these in your Lync Server event log sometime between May and June of 2017, while at the same time people can’t share PowerPoints or do Whiteboards / Q&A…
Then you’ve fallen victim of a known issue with the May 2017 .NET Framework Security and Quality Rollup affecting the Web Conferencing Service.
Luckily, Microsoft detailed two workarounds, one involves getting a new Edge Internal cert, and another involved a registry change. Here’s the details: