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Editorial Team
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Asked: 2026-06-10T20:43:42+00:00

I have an Outlook Add-In that’s installed from a web server. This add-in works

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I have an Outlook Add-In that’s installed from a web server. This add-in works the same way as all other add-ins I’ve worked on in Office.

  1. setup.exe is downloaded and executed
  2. all prerequisites (.Net framework and such) are verified
  3. vsto file is downloaded

So far, everybody installing this add-in has had no problems (this add-in has been in use for about a year). We have a new customer installing it on Windows 7 and we’re seeing our first issue. Here’s the error:

There was an error during installation:
Downloading http://<path and filename>.vsto did not succeed

***************** Exception Text******************
System.Deployment.Application.DeploymentDownloadException: Downloading http://<path and filename>.vsto did not succeed
System.Net.WebException: The remote server returned an error: (407) Proxy Authentication Required. ---> System.ComponentModel.Win32Exception: The token supplied to the function is invalid
at System.Net.NTAuthentication.GetOutgoingBlob(Byte[] incomingBlob, Boolean throwOnError, SecurityStatus& statusCode)
at System.Net.NTAuthentication.GetOutgoingBlob(String incomingBlob)
at System.Net.NtlmClient.DoAuthenticate(String challenge, WebRequest webRequest, ICredentials credentials, Boolean preAuthenticate)
at System.Net.NtlmClient.Authenticate(String challenge, WebRequest webrequest, ICredentials credentials)
at System.Net.AuthenticationManager.Authenticate(String challenge, WebRequest request, ICredentials credentials)
at System.Net.AuthenticationState.AttemptAuthenticate(HttpWebRequest httpWebRequest, ICredentials authInfo)
at System.Net.HttpWebRequest.CheckResubmitForAuth()
at System.Net.HttpWebRequest.CheckSubmit(Exception& e)
--- End of inner exception stack trace ---
at Microsoft.VisualStudio.Tools.Applications.Deployment.ClickOnceAddInDeploymentManager.InstallAddIn()

Obviously, I cannot duplicate from my location. My gut is telling me this customer’s network security is causing an issue (my understanding is that they have a series of firewalls and such), but their Windows XP machines are having no problem installing.

Anybody see anything I’m missing?

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1 Answer

  1. Editorial Team

    In the end, the answer wasn’t too difficult. What was happening is that, obviously, the user is behind a Proxy Server that’s blocking the download of the VSTO file, as it didn’t have the Proxy Credentials.

    So, I’ve had to make an exception for these users, sending them all the files in a zip that they can install from.

    Note that users that run into this MUST run a command to clear the ClickOnce app cache before proceeding; it looks like the mere attempt to install this was leaving some residuals behind that was causing a problem. If you don’t already know, you can clear the app cache by running this command from the command prompt (or creating a .bat file containing):

        rundll32 dfshim CleanOnlineAppCache
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