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I understand that users couldn’t sign in Lync with the “a personal certificate error”. Is the full error message “There was a problem acquiring a personal certificate required to sign in”?
If it is, to troubleshoot the issue, you can refer the KB article: http://support.microsoft.com/kb/2604176.You can also refer the article:http://office.microsoft.com/en-us/communicator-help/troubleshooting-lync-sign-in-errors-administrators-HA102759022.aspx
To further troubleshoot the issue, I would like to collect the following information. Have you added your domain to Office 365 recently?Have you updated user’s sign in address?Are the users created from Office 365 directly?
I would like to follow up with the question you posted previously. Is the issue resolved?