Even as I type this, the password pop-up interrupts/impedes my typing every 60 seconds. It has been doing this for at least 6 months. Until Microsoft graces us with a solution, I am forced to shut down Outlook so I can get my work done. Or use webmail or Mozilla Thunderbird....
Use webmail and make sure the mail you’re looking for is still there. (https://mail.office365.com) If so, you might be seeing the effect of a setting in Outlook to only sync the last 1-2 years of messages. Click on File / Account Settings and double-click on the account. You c...