This update fixes some issues related to Office 365 / Lync Online. No changes related to Lync on premise deployments.
- 2543532 Credential prompts occur when you sign in to Lync 2010 by using the Lync Online service
- 2543531 Lync 2010 does not authenticate after you resume the computer from hibernation
Details and download here: http://support.microsoft.com/kb/2540951
This replaces the earlier April CU2 client update, so if you haven’t got to that yet just use the latest update.