Sometimes Lync 2010/2013 client cannot login to the server. You may be sure that user account enabled in Lync Server, all caches are cleared, but anyway Lync client give error.
Lync 2010 login issues
Lync 2010 have less issues, so we'll deal with it first.
First af all, you must clear local cache in user profile: C:\Users\<username>\AppData\Local\Microsoft\Communicator\. You can completely delete this folder.
Second, Lync 2010 have issue, when user try to login before it was enabled in Lync Server, something in client goes wrong and after it was enabled in Server, client still cannot login. The same issues can occur when some fields in Active Direcotry user account is changed.
To fix this issue you mush reset account settings in Lync 2010 client: