From time-to-time, I do come across Lync clients with this small exclamation mark in the lower right corner indicating an Exchange Connection error:
There is plenty of information and troubleshooting guides related to this problem on the Internet, but most of them deal with central problems in the Lync/Exchange topology addressing all the users housed on that topology.
I do have a small dirty trick that will help 90% of the issues where this happens on a single computer/user:
Skype for Business HKEY_CURRENT_USERSoftwareMicrosoftOffice16.0Lync
Start Outlook and Lync
Now it’s time to grab a cup of coffee and wait 5-10 minutes for the Lync client to establish contact with your Exchange environment. You will see this in Lync Configuration Information (rightclick on Lync in System Tray):
Lync waiting for Exchange connection
If you have separate mail and sip domains, it could be necessary to set the DisableEmailComparisonCheck variable: