Clear Skype for Business or Lync Temporary Cache Files

Follow instructions here: https://www.cosocommunity.com/hc/en-us/articles/200887737-How-to-Clear-Lync-Temporary-Cache-Files

In path for the location where temp files are stored, use \%AppData%\..\Local\Microsoft\Office\16.0\Lync\ for SfB.
use \%AppData%\..\Local\Microsoft\Office\15.0\Lync\ for Lync.