Outlook on the Web startup failure – HTTP 500 (StartupData)

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Hi @Spence Novick, 

Thank you for posting your question in the Microsoft Q&A forum. 

Based on the details you shared that Outlook on the Web is not loading for a mailbox and returns HTTP 500 during StartupData while the client shows an unexpected end of JSON input. I truly understand and appreciate your effort in capturing the UTC timestamp, client version, and error trace because this information enables precise correlation with backend telemetry. 

From a service perspective, the client expects a complete JSON payload from the StartupData request to finish initialization. When the response body is empty or truncated, JSON parsing fails and the startup sequence cannot complete, which aligns with the symptoms you observed. Contributing factors can include a transient service condition, a mailbox configuration read that does not complete, or a network intermediary that interrupts the response before delivery.  

Below are some workable options that might be the most appropriate for your current situation:  

1/ Isolate and capture diagnostics for correlation 

• First, reproduce the issue once in a private browsing session, then sign in with a different browser profile and test again from an alternate network path such as a mobile hotspot. This sequence helps separate client cache and proxy effects from service behavior. 
• Next, collect a single clean HAR trace and the browser console log during one reproduction. In the Network view, confirm whether the StartupData call returns an empty or partial body, then share the HAR, the console log, and the exact UTC time from the error banner so we can align them with backend logs. 
• Finally, include the Client ID and Session ID from the failure page with the files to enable end to end correlation through Exchange Online telemetry.  

2/ Apply targeted Microsoft 365 checks for the affected mailbox 

• Begin by verifying that Outlook on the Web is enabled for the user and that a valid Exchange Online license is assigned, since similar StartupData failures can occur when OWA access is disabled or a mailbox is not licensed. 
• Additionally, temporarily remove organization deployed add ins for the user and retest. This validation rules out an add in initialization that interferes with StartupData processing. 
• Moreover, as a controlled test, assign a standard OWA mailbox policy with default settings, wait for propagation, and try again. If the issue clears, revert and refine the original policy to identify the conflicting setting.  

3/ Validate the network and service path 

• First, test from a path that bypasses SSL inspection and content filtering for Microsoft 365 endpoints so the response is delivered unaltered. This step addresses scenarios where intermediaries truncate or strip response bodies that the client expects to parse as JSON. 
• Next, review Conditional Access policies that apply to Outlook on the Web and, for a short window, exclude the affected user to confirm whether policy evaluation interrupts StartupData calls. 

For your reference, that you can concern:

• Outlook Error: 500 - Microsoft Q&A 
• Resolving “Unexpected End of JSON Input” Error: A Complete Guide | by UATeam | Medium 

Please note that this information is being provided by the moderator solely for your convenience. The referenced sites are not managed or overseen by the moderator, and therefore we cannot guarantee the accuracy, security, or suitability of any software or content available there. We recommend reviewing all information carefully and ensuring you understand any associated risks before following suggestions from the provided link.   

4/ Contact IT administrator:   

Since your account is managed by your organization, please contact your IT administrator to review your permissions and policies. If the issue persists after these checks, ask your IT administrator to submit a support request directly to Microsoft Support team.        

They can raise a support ticket by visiting: Get support - Microsoft 365 admin | Microsoft Learn  

As community moderators, we appreciate your understanding that our access to internal development details is limited. Our primary role is to guide users toward the appropriate resources and support channels. While we may not have visibility into deeper backend analysis, we’ll continue doing our best to support you within the scope of our responsibilities.      

I hope this information is helpful. Please follow these steps and let me know if it works for you. If you have any updates regarding the issue, please feel free to share them with me.        

Thank you for your patience and your understanding. I look forward to continuing the conversation.  

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