Troubleshooting Microsoft Outlook Add-In (Legacy)
  • 24 Nov 2023
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Troubleshooting Microsoft Outlook Add-In (Legacy)

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  • PDF

Article Summary

The Slate Outlook Add-In is a legacy tool and is provided in an as-is state. Below are outlined a list of common problems and their solutions. 


For universal support across operating systems and cloud based email services, we recommend the use of the Email to Slate Gateway.

Technical Specification:

The Slate Outlook Add-In operates using Microsoft Outlook's built-in browser which operates as a "lite" version of Internet Explorer, regardless of the main/default system browser configuration. As a IE-lite version, it uses the system configurations for Internet Explorer and some of these will become relevant below.

When operating normally, the Slate Outlook Add-In's workflow starts by navigating to, which prompts for the domain of your Slate instance (e.g. and then redirects the Slate Outlook Add-In to that specific database's outlook service (e.g., along with the data from the selected email. At this point, the user will need to authenticate with Slate in the same way that would be experienced when accessing Slate through a normal browser.

While the Slate Outlook Add-In remains loaded as part of the current Outlook session, the chosen domain and user's Slate session values are stored; however once Outlook is closed the cache is cleared forcing the user to login next time.

Specific Error Messages

The following feature couldn't be installed: .NET Framework 3.5

The Slate Outlook Add-In is based on Microsoft's .NET Framework 3.5 and when running the setup file it will attempt to install the prerequisite software. When this fails you may need to be enable the Feature manually via the computer's Control Panel. (

Should this not resolve your problem, uncheck the Feature from the Control Panel and deploying .NET Framework 3.5 by using Deployment Image Service and Management (DISM).

The login page doesn't show after entering the domain name

Sometimes this might present as a simple white screen, while other times you might see a "Navigation to the webpage was canceled" message. This is indicative of a failure by Outlook's browser to cleanly handle the redirection to the authentication service. We can start by adding the various domains used as Trusted Sites list. In some cases this might cause the SSO to try and use a browser popup to prompt for credentials which isn't supported by the Outlook browser, and will require a modification to the Security User Authentication.

Add Slate Domains as Trusted Sites

  1. Close Outlook and any Internet Explorer windows you might have open

  2. Open the Control Panel

  3. Open the Internet Options tool

  4. Change to the Security Tab

  5. Select the Trusted sites zone

  6. Click on the Sites button

  7. Enter as URL

  8. Enter your Slate domain name (e.g.

  9. Enter your SSO domain name (e.g. 

Configure User Authentication

  1. Close Outlook and any Internet Explorer windows you might have open

  2. Open the Control Panel

  3. Open the Internet Options tool

  4. Change to the Security Tab

  5. Select the Trusted sites zone

  6. Click on the Custom level... button

  7. Scroll to the bottom of the list and locate the User Authentication section

  8. Within this selection select the Automatically log-on with current username and password option (Note: This option is selected by default within the Intranet zone, and as such you might want to add the Trusted Sites from above to the Intranet zone list instead.)

Reset Internet Explorer

  1. Close Outlook and any Internet Explorer windows you might have open

  2. Open the Control Panel

  3. Open the Internet Options tool

  4. Change to the Advanced Tab (figure 1 below) and click the Reset... button

  5. Check the Delete personal settings and click on "Reset" (figure 2).

Figure 1: Advanced Tab

Figure 2: Reset example

An error occurred trying to download ''

This error can occur during the installation of the Slate Outlook Add-In tool, the corrective actions for which are to start by uninstalling the Slate Outlook Add-In followed by a reset of Visual Studio Tools for Office (VSTO). You can then attempt an automatic installation of the Add-In using the setup file, however we've seen the best results by manually installing and registering the Slate Outlook Add-In.

Reset Visual Studio Tools for Office (VSTO) Configuration

  1. In Windows Explorer, navigate to the VSTO 10.0 folder which is the path specified in the error message, e.g., C:\Program Files (x86)\Common Files\Microsoft Shared\VSTO\10.0

  2. Locate the file VSTOInstaller.exe.config and, if present, rename it to VSTOInstaller.exe.config.old (Ignoring the warning from Windows about the file becoming unusable)

  3. Visit

  4. Download the installation file

  5. Execute the downloaded file to start the installation process

Manually Install the Slate Outlook Add-In

  1. As an administrator, open a command prompt

  2. Navigate to the VSTO install folder "C:\Program Files (x86)\Common Files\Microsoft Shared\VSTO\10.0"

  3. Execute the following command "vstoinstaller /i"

Manually Register the Slate Outlook Add-In

  1. Create a new Registry Key of “Slate.Outlook” in the following location:

  2. Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Addins\

  3. Add the following values within the new Key:

    • (String Value) "Description" = "Slate.Outlook"

    • (String Value) "FriendlyName" = "Technolutions Slate"

    • (DWORD Value) "LoadBehavior" = dword:00000003

    • (String Value) "Manifest" = ""

Specified argument was out of the range of valid values.

This can often be caused by a corruption of the local ClickOnce application cache (ClickOnce being the technology Slate uses to install the Slate Outlook Add-In), and if so the solution would be as follows:

Open a command prompt and run the command: rundll32 dfshim CleanOnlineAppCache rmdir /s %localappdata%\apps\2.0

If an "Access is denied" message appears, reboot the computer and then open a command prompt immediately upon restarting (prior to accessing Slate) to ensure that there are no files actively in use.

Please note that this action might require a local Administrator depending upon your campus' security and Group Policy configuration.

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