Assume that you set the automatic logon through Windows Credentials for Microsoft Lync 2013 (Microsoft Skype for Business) on a non-domain-joined computer. (You can access the Credential Manager item in Control Panel.) When you log on to the client that has December 8, 2015 security update installed, you are prompted for credentials.
To fix this issue, install the February 9, 2016 update (KB3114732) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.