Sometimes a new capability is introduced in an update and even if you go through every detail you may still miss out on the little gem.
This is the case with the Lync Sign-In logs..allegedly introduced around CU3 or CU4 (not sure cause I only just found it)

Simply to provides you with the Sign-In logs of unsuccessful sign-in attempts so that you can troubleshoot the process.

How to use it

CTRL and Right Click on the Lync icon in your task bar and select Sign-in Logs

If the window states “no sign-in data available” that’s simply because the sign-in process was successful.
To collect the logs click on the Copy to Clipboard button and paste that into a text editor of your preference

In my example here you will see 2 reported errors.
The first states a trust issue and the second mentions that the client isn’t authorized.

The underlying cause, invalid credentials.