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Thank you for sharing all the troubleshooting details. I can understand how challenging this must be when the issue affects only certain users and behaves differently across browsers and upload methods.
I noticed that you are already working with Microsoft Support, so I recommend continuing to work closely with them and providing any logs, screenshots, or test results they request. Since they have access to additional diagnostic tools and internal resources, they are in the best position to determine whether the problem is related to your environment or backend configuration, or whether it is a known Microsoft issue. They can also address specific questions about your case based on their investigation.
People in this forum can provide general recommendations and troubleshooting suggestions, but we do not have access to your account, environment, or the diagnostic tools needed to confirm the root cause. Since Microsoft Support is already investigating the issue, continuing to work with them would be the most appropriate next step.
I apologize for the inconvenience and thank you for your understanding.
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