Symptoms
In a Skype for Business hybrid environment, when you sign in to the Skype for Business client after switching the network, you may find you're prompted to enter Exchange credentials. This issue occurs when the Skype for Business client uses basic authentication to sign-in but Exchange Online uses modern authentication (ADAL).
Resolution
To fix this issue, install one of the following updates:
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For Skype for Business 2016, install the July 10, 2018, security update for Skype for Business 2016 (KB4022221).
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For Skype for Business 2015 (Lync 2013), install the July 10, 2018, security update for Skype for Business 2015 (Lync 2013) (KB4022225).
Note Lync 2013 was upgraded to Skype for Business in April 2015.
