Hi,
In your case, we need to collect network packet capture to find the cause. However, analysis of network traffic is beyond our forum support level and due to forum security policy, we have no such channel to collect user log information. So we recommend you open a case with MS Professional tech support service, they will help you open a phone or email case to Microsoft, so that you would get a technical support on a one-to-one basis while ensuring private information.
Here is the link:
https://support.microsoft.com/en-us/gp/customer-service-phone-numbers
Best Regards,
Candy
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