Had an issue with mailboxes migrated to Office 365 while using OnPrem Lync/Skype.
Lync showed a constant “Enter Credentials” dialog:
The KB appears to have fixed it: Office 2013 and Lync 2013 periodically prompt for credentials to SharePoint Online, OneDrive, and Lync Online
A simple registry key that you can rollout with a GPO.
flaphead.com 