Hi curtis.aloia, Office365 redirects among numerous servers, so the problem may be during that hand-off.

If you start with this general URL:

https://outlook.office.com/mail/inbox

Then you should get redirected to a long URL starting with

https://login.microsoftonline.com/common/oauth2/authorize?

Does the error appear right away at that point, or after you submit the form?

I can't tell what the "reply URL" in your error text refers to. Part of the URL is an encoded version of that starting URL --

redirect_uri=https%3a%2f%2foutlook.office.com%2fmail%2f

-- but there are many, many other parameters which are not easily understood.

Judging from a Microsoft support thread, it looks like sometimes the error may have additional detail. For example, "More details: Reply address did not match because requested address did not have a trailing slash."

https://docs.microsoft.com/en-us/answers/questions/353/aadsts50011-the-reply-url-specified-in-the-request.html

Anything like that would be useful to know.

Hi curtis.aloia, Office365 redirects among numerous servers, so the problem may be during that hand-off. If you start with this general URL: https://outlook.office.com/mail/inbox Then you should get redirected to a long URL starting with https://login.microsoftonline.com/common/oauth2/authorize? Does the error appear right away at that point, or after you submit the form? I can't tell what the "reply URL" in your error text refers to. Part of the URL is an encoded version of that starting URL -- redirect_uri=https%3a%2f%2foutlook.office.com%2fmail%2f -- but there are many, many other parameters which are not easily understood.

This does not happen right away, but when it redirects to the https://login.microsoftonline.com/common/oauth2/authorize? address.

Judging from a Microsoft support thread, it looks like sometimes the error may have additional detail. For example, "More details: Reply address did not match because requested address did not have a trailing slash." https://docs.microsoft.com/en-us/answers/questions/353/aadsts50011-the-reply-url-specified-in-the-request.html Anything like that would be useful to know.

There is nothing indicating more details, but fortunately, this error is now an incident on my Exchange Service Health report. I will keep you posted on what Microsoft can offer as a solution.

Thank you.

About 45 minutes before you posted, another user reported a similar error message:

https://support.mozilla.org/questions/1286896

Maybe there's a minor change somewhere in the system.

About 45 minutes before you posted, another user reported a similar error message: https://support.mozilla.org/questions/1286896 Maybe there's a minor change somewhere in the system.

The incident report returned: "Additional investigation determined that a recent change contained a code regression that resulted in authentication failures within the Outlook on the web. We've disabled the deployment and are currently in the process of rolling the affected change back to mitigate impact."

After this update, the issue has been resolved.

Thank you for all of your help!