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.
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: