Since a few days users are facing issues with Outlook search in connection with an Exchange mailbox at Office365.com. This is a known issue for which Microsoft wants to roll out a fix.
I just stumbled uppon the following Tweet mention this isse with office365.com.
You’re using #Exchange mailboxes within #Office365 and since a few days you have trouble using the built-in Outlook #search? Microsoft is (still) rolling out a fix for this problem. (screenshot of Status tab in O365 Admin panel) pic.twitter.com/DuHlMulHW1
— Oli’s HelpDesk (@OlisHelpDesk) November 4, 2019