This was a known issue and it should be solved after the following Lync client update 15.0.4551.1005 http://support.microsoft.com/kb/2825630/en-us,Make sure that you have installed latest Lync client updates and check if the issue still exist.
Also you can solve the problem by the following:
Ø Rename the Cache in C:\Users\<Name>\AppData\Local\Microsoft\Communicator. Then log in to the Lync Client, add a custom “What Happening” message and then delete it. It will be reset the field back the original “Whats happening today” message
Ø I you are uding Lync wendows app store macksure you installed the latest updated Lync client from windows app store