File this under tips and tricks:
You are an Office 365 subscriber and trying to launch the Lync or Skype for Business client application. When attempting to login you receive the following error…
This happens due to server authentication failure. But you can set the server settings manually. Go to Tools -> Options -> Personal -> Select Manual Configuration -> Enter: sipdir.online.lync.com:443 -> Click OK -> Click OK on the Options window.
Go back to the login screen and enter your credentials. Usually this is your email address.
Now you should be able to successfully login.
If you still encounter issues, try using the Microsoft Remote Connectivity Analyzer.