Outlook keeps asking for sign-in
Since upgrading to iOS 12, Outlook has periodically (about twice a week) required me to sign-in to my IMAP accounts. It will not sync or do anything unless I sign-in yet again. This happens on both my iPhone and iPad. The iPhone is a Xs Max - so for it, this has been going on since day 1. My prior iPhone 6 Plus never had this problem. The iPad Air never had this problem until I upgraded to iOS 12.
Interestingly, when one device requires signing-in, the other one also requires it. They are in sync with this! It's annoying, because I have a password that is rather difficult to remember and to type on the iOS keyboard.
Does anyone have any idea what's going on and how to solve - especially if this is an Outlook problem?
iPhone XS Max, iOS 12.0.1