You're prompted for Exchange credentials when you switch to internal network for Skype for Business

Assume that you sign in to the Skype for Business client by using an external network. When you switch from the external network to an internal network, you may be repeatedly prompted to provide a user name and a password to connect to Exchange. 

This issue may occur in Microsoft Skype for Business 2016 and Skype for Business 2015 (Lync 2013).

A timing issue leads to reading the EnableRestoreOAuthUsedKeyWhenUsingCachedWebTicket value to false, even when it was set correctly by the administrator.

To fix this issue, install one of the following updates:

• For Skype for Business 2016, install the February 6, 2018, update (KB4011662).
• For Skype for Business 2015 (Lync 2013), install the February 6, 2018, update (KB4011678).

Note Lync 2013 was upgraded to Skype for Business in April 2015.

Even though the update is enough to fix this issue, there is also a GPO setting that you may need for testing:

Create a GPO to add a new DWORD registry key EnableRestoreOAuthUsedKeyWhenUsingCachedWebTicket and set the value of the key to 1 under one of the following paths:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Lync\SipID
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Lync\SipID