Exchange Web Services Client No Longer Using Windows Credentials to Authenticate

My EWS client has stopped authenticating against our on-premises 2013 exchange server. The client is written in C++ using the Win32 API. Attempts to call WinHTTPSendRequest are now returning a 401 status code and if I manually set credentials (WinHTTPSetCredentials) the request works. Why does it no longer automatically make use of the currently logged on users credentials which should happen when WINHTTP_OPTION_AUTOLOGON_POLICY is set to WINHTTP_AUTOLOGON_SECURITY_LEVEL_LOW?

If I look at the IIS logs I can see only one entry for the connection where as older logs show 3 entries which would be consistent with the client automatically authentication the connection.

IIS Log Entries

September 11th, 2015 5:04am

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