Configuring preferences and options in Outlook.com
I also got the same problem.
I did change some aliases on my Microsoft account a few days before it started happening (not sure if it is related).
Changing browsers did nothing.
Also, I noticed that when I select the Sync Email settings, there is a request a 500 response code.
I believe the problem is actually that I removed all my connected accounts that used to be able to add on these settings, but there was no option to add them again. Now they are null(?)