Assume that you start to sign in Microsoft Skype for Business 2015 (Lync 2013), and there's a metadata download failure that causes later requests to fail. When the client retries the request, the MEX document download fails without trying to contact the server. However, if you contact the server to fetch this document to get the metadata information successfully, you can sign in the client.
To fix this issue, install the December 6, 2016, update (KB3127976) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.