The user accounts in Outlook and Microsoft Lync do not match. Please make sure you are signed in to both application with the same user name and privileges.

During a recent test before rolling out a mass SIP URI change at a customer's site on Lync 2013, I have come across this issue when trying to schedule a meeting:

Bit of background: current SIP URI and email did not match (both the username and the domain portions were different), and the change was planned with the purpose to match them as per established best practice, so the error left me a bit puzzled for starters.
After some analysis, I found that the client policy assigned to the user had the following parameter set:

DisableEmailComparisonCheck: FALSE

I didn't expect this would break Lync/Outlook interoperability now that SIP URI matched email address, anyway, this is what fixed the issue for me:

Set-CsClientPolicy -Identity Global -DisableEmailComparisonCheck $null