I have a wcf web service deployed inside the isapi folder of a sharepoint 2013.
Sometimes the web service go down and an iisreset seems to solve the problem.
The error message complains about a 'ntlm' authentication against a 'NTLM' authentication.
However I always log with the same user so the authentication problem seems strange to me.
Logs: SPSecurityContext: Get trust client failed with exception: System.IO.FileLoadException: Could not load file or assembly 'Microsoft.IdentityModel.Extensions, Version=126.96.36.199, Culture=neutral, PublicKeyToken=69c3241e6f0468ca'...
Application error when access /_vti_bin/FileSystem.svc, Error=Provider type not defined. (Exception from HRESULT: 0x80090017)
Non-OAuth request. IsAuthenticated=True, UserIdentityName=, ClaimsCount=0
SPRequest.PreInitServer: UserPrincipalName=, AppPrincipalName= ,bstrAbsoluteRequestUrl=...