Symptoms
When you switch between inside corporation network and outside corporation network to sign in to Microsoft Skype for Business 2015 (Microsoft Lync 2013), this issue occurs.
Cause
This issue occurs because the stale proxy cache is still used in the new network environment.
Resolution
To fix this issue, install the April 4, 2017, update (KB3178731) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.
