Outlook Error Codes

How to Fix Remote Endpoint Error HTTP 400 in Outlook

Rana Garcia Sep 29, 2026 868 views

Question details

The user needs to resolve an HTTP 400 remote endpoint error that prevents approval requests from being processed in Microsoft Outlook.

How to Fix Remote Endpoint Error HTTP 400 in Outlook

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Attempting to process or interact with an approval request directly within the Outlook email client.
Observed behavior: Outlook fails to process the request and returns an HTTP 400 remote endpoint error, likely due to faulty authentication tokens, cached data conflicts, or add-in interference.

Before you start

Before attempting these fixes, verify that your device is connected to a stable network and check if the approval works when logging into Outlook on the web (OWA).

Solution 1Recommended

Create a New Outlook Profile (Classic Outlook)

Creating a new profile is the most effective way to rebuild corrupted local configurations and refresh authentication tokens that cause HTTP 400 errors.

If you are using the Classic Outlook desktop client, your local profile may hold outdated or corrupted tokens preventing the server from accepting your approval request.

Close the Application

Ensure that Microsoft Outlook is completely closed and not running in the system tray.

Open the Profile Manager

Press the Windows Key + R to open the Run dialog box. Type outlook.exe /profiles and press Enter.

Add a New Profile

In the Mail Setup window, click the 'New' button, enter a name for the new profile, and click OK.

Sign In and Configure

Follow the on-screen prompts to sign in with your email credentials and set the new profile as your default before relaunching Outlook.

Data Safety: Creating a new profile does not delete your emails. Outlook will securely resync your data from the server once the new profile is active.

Solution 2

Disable Conflicting Add-ins and Clear Cache

Third-party add-ins or corrupted temporary cache files can intercept network requests and trigger endpoint errors.

Solution 3

Test Using an Alternative Outlook Client

Isolating the issue helps determine whether the error is specific to your desktop installation or a broader server-side policy issue.

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Frequently Asked Questions

What causes an HTTP 400 Bad Request error in Outlook?

An HTTP 400 error in Outlook typically occurs when the server cannot process a request due to invalid syntax, corrupted authentication tokens, conflicting third-party add-ins, or stale cached data stored on the local client.

How do I know if my Outlook add-ins are causing endpoint errors?

You can isolate add-in issues by launching Outlook in Safe Mode (typing outlook.exe /safe in the Run dialog). If the approval request goes through without an HTTP 400 error, an active add-in is likely the culprit and should be disabled in the settings.

Is it safe to delete the Outlook RoamCache folder contents?

Yes, deleting the files inside the RoamCache directory is entirely safe. It only clears temporary cached data like autocomplete lists and temporary network files, which Outlook will automatically rebuild upon its next launch.

Microsoft Office Error Codes