Solving "Request failed with status code 400"

Outlook Add-Ins run inside Microsoft Outlook using a component called "Microsoft Edge's WebView2" (basically a smaller version of the Edge Browser you might use as a daily. This setup uses browser-like technology to render content. Over time, the cache and cookies stored by WebView2 can become outdated or corrupted, especially after changes in your Sidebar settings or network environment.

These issues can lead to the "Request failed with status code 400" error, which usually indicates a bad request between the client (your Sidebar) and the server. Clearing the cache and cookies forces a fresh load, resolving most communication and display problems.

Step 1: Check If WebView2 Is Installed

This guide applies only to systems using MS Edge WebView2. To check if it's installed:

• Click the Start menu and go to Settings > Apps > Apps & Features.
• In the search bar, type WebView2.
• Look for WebView2 Runtime in the search results.

If you are not using WebView2 then the contact support instead, but otherwise continue:

Step 2: Clear WebView2 Cache and Cookies

Follow these steps to clear cached data and force a full Sidebar reload:

• Close all Microsoft Outlook windows.
• Open File Explorer (click the folder icon on your taskbar).
• Copy and paste the following path into the address bar and press Enter:

%LOCALAPPDATA%\Microsoft\Office\16.0\WEF\

• Find the folder named webview2.
• Right-click it and select Delete, or press the Delete key on your keyboard.
• Reopen Microsoft Outlook, start MailMerge365 again and try to login now.

Still Having Issues?

If the problem persists after clearing the cache, reach out to our support team. We're here to help!