Assume that you are using Microsoft Skype for Business 2016 through a proxy in an internal network. When you switch the connection to an external no-proxy network, you cannot sign in the client. This issue occurs because the proxy caches aren't cleared, and the client continues to try the old proxy that's no longer available.
Note This issue occurs prominently when the internal proxy is available externally.
To fix this issue, install the October 10, 2017, security update for Skype for Business 2016 (KB4011159).