SharePoint High Trust Application - stopped being trusted - 401

Question

We've got a high trust application which ok, in the mornings seem to 'not work' but then after the first hour was fine. But now it just blankly doesn't work with the non helpful.

System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   at System.Net.HttpWebRequest.GetResponse()
   at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute()
   at Microsoft.SharePoint.Client.ClientRequest.ExecuteQueryToServer(ChunkStringBuilder sb)

Any ideas?

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Edit

Ok, so bit more information, we managed to plug in fiddler into the solution using the lines

<system.net>
  <defaultProxy>
    <proxy usesystemdefault="False" bypassonlocal="False" proxyaddress="http://127.0.0.1:8888" />
  </defaultProxy>
</system.net>

These showed the results coming back but labelled 401.

Answer 1

In the end we ran

Set-SPAppPrincipalPermission -Site $web -AppPrincipal $appPrincipal -Scope Site -Right FullControl

and then did a iisreset on the share point server.

For us we have 3 iis servers for the high trust solution, load balanced and then 2 sharepoint frontend servers. We did bypass the loadbalencer for the 2 sharepoint frontends -but we'll probably bring that back into the fray for dependency.