Unable to start MSFS - in the middle of sim boot-up I get a "We couldn't sign you in to Xbox Live" notice, then everything stops.

I checked the status of the Xbox system online - it states that all services are working.

OK - I removed the Xbox live component from windows, then downloaded and reinstalled the Xbox app.  The gamebar ran fine but the Live component still wouldn't start.

Finally, I chose "use a different account" (even though I don't have another account) and told it to use my regular account.  It asked for my pin, I entered it and voila! Xbox Live is running again.  Weird.

If I don't post on this again, then FS started successfully.

interesting.  I got that warning for the first time today.  It gave me the option to retry, which I did and all was OK.  I'm guessing that x box live may have had an outage at some point.

Yeh - every Retry failed for me and a check of the Xbox servers showed everything was running.  I think you're correct - there was an outage that knocked us off and required a re-login (in my case).  The only way I was able to initiate login access was to select "use a different account".