Symptoms
Assume that you enable the passive authentication for the web services of the Front End Server pool in Microsoft Skype for Business Server 2015. When you use the Skype for Business client to access the external web services, a credential prompt (sign-in dialog box) is displayed even if the NTLM authentication protocol is disabled.
Resolution
To fix this issue, install the December 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015, Web Components Server.