Remote Endpoint error (HTTP 400)

The HTTP 400 error you are encountering when interacting with an approval prompt in Outlook typically indicates that the request sent to the server is malformed or cannot be processed due to some issue with the request format. This could be related to several factors, such as incorrect parameters or issues with the server's ability to parse the request.

To troubleshoot this issue, consider the following steps:

1. Check the Request Format: Ensure that the request being sent is correctly formatted and adheres to the expected parameters.
2. Review Add-ins: Sometimes third-party add-ins can interfere with Outlook's functionality. Try disabling any add-ins to see if that resolves the issue.
3. Clear Cache: Clearing the cache in Outlook or your web browser (if using Outlook on the web) can sometimes resolve unexpected errors.
4. Update Outlook: Ensure that your Outlook application is up to date, as updates may contain fixes for known issues.
5. Network Issues: Check your network connection to ensure that there are no interruptions that could be causing the error.

If the problem persists, you may need to consult further documentation or support resources specific to your environment or the approval prompt functionality you are using.

Hi @BusinessIntelligence,

Welcome to Microsoft Q&A, and thank you very much for reaching out to us.

Based on your description, could you please confirm which Outlook client you are currently using: Classic Outlook, New Outlook, or Outlook web?

Also, does this issue also occur when you using the same approval prompt through another Outlook client, and did the problem start recently or has it been occurring for some time? This information will help me determine whether the issue is server‑side or related specifically to the Outlook client.

I tested using a Power Automate approval flow on my end, and the approval prompt works correctly in Classic Outlook.

If possible, could you share how your approval prompt is created so I can better understand the scenario?

In the meantime, you can try the troubleshooting steps suggested by the Microsoft Q&A assist. If you are using Classic Outlook, I recommend creating a new Outlook profile to see if it able to solves your issue. A new profile rebuilds authentication tokens and local configuration data, which can help resolve issues caused by corrupted or outdated profile information.

To create a new Outlook profile, start by closing Outlook. Press Windows + R, type outlook.exe /profiles, and then press Enter. In the Choose profile window, click Options, then select New to create a new Outlook profile. Once created, choose Set as default profile to ensure this profile is used when Outlook starts.

I hope this helps, looking forward to your response.

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