We have been working with an issue in Lync 2010 clients where users were not able to sync their outlook calander information. After checking so many parameters and testing we found that issue is with one Hotfix KB2533999 which was causing issue.
To resolve this issue we had to remove this hotfix and run without patch and it fixed the issue.
There is also a hotfix KB2540951 available which would address this issue. You can install this patch on top of any previous patch if you not willing to remove.
All about Microsoft 365 & Teams 
I tried installing the KB2540951 fix but this did not work.
I also noticed that there was no KB2533999 installed.
Any other ideas would be greatl appreciated!
Hi ADZ,
Sorry for late response as i was out of town.Please remove patch 25339999 from clients and then test it.
Hi ADZ,
Is it happenming for all employee ?, I have seen for some users only and can be fix by re-install Lync clinet (no Patch). If still you have issue then please post question in below forum where we will help you in time 🙂
http://social.technet.microsoft.com/Forums/en-US/ocsclients/threads