Assume that you try to join a meeting from the Skype Meeting Add-in for Microsoft Office 2013 in Microsoft Outlook. In this situation, the client can't sign you in. When you are prompted to select to rejoin the meeting through Lync Web App, you may find that the meeting still can't be joined.
This issue occurs because there's an extra "?" character in the URL for joining a meeting through Lync Web App.
To fix this issue, install the November 7, 2017, update (KB4011255) for Microsoft Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.