When you Join a meeting in Lync 2013 and you also have installed the Lync MX App for Windows 8 then Windows asks you which application you want to use to join meetings.
When Windows asks you which program you have the option to select “Lync” which is actually the Lync MX App for Windows 8 and “Microsoft Lync” which is Lync 2013.
Windows only asks this question once so if you accidentally selected the “Lync” App for Windows 8, you are stuck with this app and does not provide all the functionality.

Meeting joined with Lync MX App for Windows 8

If you want to change this back to Lync 2103 or switch between “Lync” & “Microsoft Lync” then you need to go to

Go all the way down to Protocols and find LYNC15 (URL:Lync15)

Lync is selected as the default application (Lync MX App for Windows 8)

Select Change Program and Select “Microsoft Lync” to switch meeting join with Lync 2013

This should be the result

Now I can join again with Lync 2013