Hi Stratapult,
Based on your description, I would like to confirm the following information:
1. Please check if the other users can sign in this Lync 2011 for Mac client.
2. Try to sign in Lync with the problematical account in another Lync client to see if the issue will persist.
To troubleshoot the issue, please try to the following suggestions:
1. Make sure the user follow the correct steps when they sign in to Lync Online by using Lync for Mac:
(1). Start Lync for Mac.
(2). Enter your email address, user ID, and password.
(3). Next to Sign in as, select the presence status that you want to display after you sign in.
(4). If you want to save your credentials, click to select the Remember my password check box. Your credentials will be saved in the Mac OS Keychain and will work until the next time that your password changes or expires.
(5). To configure additional options for connecting to Lync Online, click Advanced. You can configure the following options:
a. Manual Configuration: Select this option if automatic configuration is failing, and specify the following values:
Internal server name: sipdir.online.lync.com:443
External server name: sipdir.online.lync.com:443
b. Kerberos authentication: Leave this option cleared. Office 365 uses the Office 365 identity platform for authentication.
2. Users may be unable to sign in unless they first use an incorrect sign-in address.
(1) Try to sign in to Lync Online by using an incorrect sign-in address. That is, a sign-in address that you know won't work. For example, \EmailAddress @contoso.com.
Note When you do this, sign-in will fail. This is expected behavior.
(2)After the sign-in fails, sign in by using the correct sign-in address. For example, EmailAddress@contoso.com.
Reference
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Troubleshoot the Lync for Mac issues: http://support.microsoft.com/kb/2629861#SignIn
Thanks,
Anna Shi