Why does my Outlook Web Add-in display startup errors and require Retry → Start before loading?

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Answer 1

Good day! Welcome to Microsoft Q&A forum!

Based on your description, I understand that your Outlook Web Add-in does eventually load and function correctly, but each time it is opened, users receive two startup error messages. The first error requires clicking Retry, and the second warning requires clicking Start before the add-in loads successfully. You're looking to understand what might be causing these startup errors and what troubleshooting steps can help identify the root cause so the add-in launches normally without user intervention.

Regarding your situation, this can sometimes be related to manifest configuration, network connectivity, authentication delays, browser security policies, resource loading problems, or initialization errors within the add-in itself.

To understand the situation better and provide you with the most suitable support, could you confirm the following information:

Is the issue occurring for: All users? A specific user? A subset of users?
Is the add-in: Sideloaded for testing? Centrally deployed through Microsoft 365 Admin Center? Published through AppSource?
Has the add-in worked correctly in the past, or has this behavior been present since deployment?
Are there any recent changes to: The add-in manifest? Authentication configuration? etc.
Have you tested the add-in in an InPrivate/Incognito browser session? If so, does the same behavior occur, or does the add-in open without displaying the Retry and Start prompts?
Does the issue occur across multiple browsers (for example, Edge, Chrome, Firefox), or is it limited to a specific browser?
Have you tested the behavior using a different mailbox/account? If yes, does the issue occur for all mailboxes or only specific users?
Have you tested the add-in on a different device or workstation? If so, does the behavior remain consistent?
Have you checked with your IT/Microsoft 365 administrator to determine whether any organizational policies, security controls, or recent environment changes could be impacting the add-in's ability to load properly?

Your confirmation would be very helpful in ensuring you receive the most suitable support!

If you’re able to share a bit more about your situation, I might be able to suggest some helpful next steps.

Kindly let me know when there are updates or if you need further assistance. Any updates you’re able to share would be really helpful. I appreciate your time and look forward to hearing how things are going!

Thank you for your time and patience.

If you have extra questions about this answer, please click "Comment".

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Rohan M 5 Reputation points

2026-07-30T06:23:17.0533333+00:00

Hi @Julie Huynh

Thank you for your response and for taking the time to review my question.

Please find the requested information below:

The issue occurs for all users who access the add-in.

The add-in is currently sideloaded for testing. We are still in the development and testing phase.

This behavior has been present since we started testing the add-in.

There have been no recent changes to the add-in manifest. We have not implemented authentication yet, so there have been no authentication-related changes either.

Yes. The same behavior occurs in an InPrivate/Incognito browser session.

Yes. The issue is reproducible across multiple supported browsers.

Yes. The same behavior occurs with different mailboxes/accounts.

Yes. The behavior remains consistent across different devices.

We are currently testing the add-in in our development environment, and we are not aware of any organizational policies, security controls, or recent environment changes that could be affecting the add-in.

Additionally, as part of our debugging, we added some console logs to trace the add-in startup process. We confirmed that the login page loads successfully, and Office.onReady() is also called successfully. The console output is:

LOGIN PAGE LOADED Office is ready { host: "Outlook", platform: "OfficeOnline" }

This indicates that the add-in initializes successfully after clicking Start, and it functions normally once loaded. However, Outlook still displays the startup errors (Retry → Start) every time the add-in is opened.

Could you please let us know if there are any additional logs, diagnostics, or specific areas (such as the manifest, Office.js initialization, or Outlook add-in lifecycle) that we should investigate to identify the root cause?

Thank you again for your assistance. I look forward to your guidance.
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Julie Huynh 2,360 Reputation points Independent Advisor

2026-07-30T15:20:56.5633333+00:00

Dear @Rohan M,

Thank you for providing the additional details and for performing such thorough testing. I appreciate the time you've taken to validate the behavior across different users, browsers, devices, and sessions.

Based on the information you've shared, the issue appears to be consistently reproducible across all test scenarios and has been present since the beginning of development. Additionally, the fact that the login page loads successfully and Office.onReady() is called after clicking Start suggests that the add-in is ultimately able to initialize and function correctly once Outlook allows it to proceed.

Since this behavior is occurring with a sideloaded add-in that is currently under development, the next recommendation depends on the ownership of the add-in:

If this is a third-party or internally developed add-in, I recommend working with the add-in developer or development team to further investigate the add-in startup sequence, manifest configuration, and any initialization logic that may be triggering Outlook's startup warnings.

If this is a Microsoft-published add-in, or if you believe the behavior is related to the Microsoft 365 platform itself rather than the add-in implementation, please open a support ticket with Microsoft so the engineering team can perform a deeper investigation using backend diagnostics. For more details, you could refer to this article: Get support for Microsoft 365 for business

Thank you for your kindness and understanding!

Why does my Outlook Web Add-in display startup errors and require Retry → Start before loading?

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