The following list includes few useful resources for troubleshooting:
Links for Troubleshooting Sign-in issues with Microsoft Lync:
Describes how to troubleshoot sign-in issues with Microsoft Lync and Office 365
Describes how to clear the credential cache that can occur on some operating systems
Describes how Microsoft Lync can cache its connection endpoints and how to resolve the issue
Describes how to troubleshoot common issues between Lync for Mac and Office 365