This event started to appear every 20 seconds or so. The Skype for Business servers had recently been patched. In the patch list was updates to .Net framework. Included in these patches is a security update that resolves an security bypass feature. https://support.microsoft.com/en-us/help/4014510/description-of-the-security-and-quality-rollup-for-the-net-framework-4 . To solve this all I had to do was add the required registry key : HKEY_LOCAL_MACHINE\Software\Microsoft\.NETFramework\v4.0.30319 – DWORD: RequireCertificateEKUs=0 and restart the “Skype for Business Server Web Conferencing” service. The fix can be applied to Lync server 2013 as well.