You can't sign in to Lync 2013 when the cached certificate is expired.
This issue occurs because Microsoft Lync 2013 uses the cached online certificate for the on-premises AutoDiscover service when Lync 2013 signs in, which fails. Then, Lync 2013 tries to achieve a new certificate from an on-premises service. The achievement fails too. Therefore, Lync 2013 sign-in fails.
To work around this issue, click Delete my sign-in infor in the sign-in window of Lync 2013 to remove the record of your logon ID and password from your computer’s memory.
To resolve this issue, install the following update:
2881070 August 2014 update for Lync 2013 (KB2881070)