Suddenly can't login to icloud in outlook IOS

Same problem, but found solution. Go to iCloud ID account page, "security", "app-specific passwords" and "generate password" for non-icloud app. Copy password and use that for the iCloud account in outlook.

Many thanks, this advice solved my problem as well! However, although I'm connected to "me" I can receive emails but can't send them : Connection to the server failed or was dropped. Error code: -3253. Do you have an idea? Many thanks!

It took me weeks to realise that my problem was two factor authentication. Turned it off then set up my Outlook on all iOS devices with no problem.

I have not turned it back on again yet, but this worked for me.

2 factor authorization did add a crimp as Apple not Microsoft told us about something so important affecting Outlook.

My problem was finally fixed by doing a rollback of Microsoft Windows 10 to settings from a month ago. Then it worked perfectly. Took about 10 hours of trying liteverything.

Little info in your post- phone model, iOS version, define "outlook iOS". Are you referring to entering your iCloud info when first activating the phone?

Answer to myself: I changed the outgoing server port to :25 - now it works.