It looks like the file is not automatically updating after DNS changes. Some users will get the following error:
You can solve this by deleting the “EndpointConfiguratation.cache” file.
Sign out of Skype for Business Click File Exit Open File Explorer Go to “C:/Users//AppData/Local/Microsoft/Office/16.0/Lyncfirstname.lastname@example.org” Select File: EndpointConfiguratation.cache and delete it. Launch Skype for Business client and sign in. When you have a older version of Office running change the 16.0 to 15.