You have two Active Directory forests in a Microsoft Lync Server 2013 environment. For example, you have forest A and forest B.
A user logs on to a computer by using an account that is from forest A.
The user signs in to Lync 2013 on the computer by using an account from forest B.
The user connects to an Active Directory Federation Services (ADFS) server in forest B.
In this scenario, the user can’t sign in to Lync 2013, and he or she receives an error message that states that the ADFS server cannot be found. Additionally, the ADFS server generates the following error message:
Unknown username and password
If a user installs Lync 2010 and then installs Lync 2013 on the same computer, the user can sign in to Lync 2013 in the scenario that’s described in the "Symptoms" section. However, if the certificate is deleted from the computer, the user can’t sign in to Lync 2013.
To resolve this issue, install the following update:
2825630 Description of the Lync 2013 update 15.0.4535.1510: October 8, 2013