Outlook Live server connection problem after upgrading to Sequoia 15.0

Safari returns - Your message can't be displayed right now. Check your Internet connection and try again.

Very often, I can't open Outlook Live or, it displays gray flashing lines instead of text. Or, Outlook opens, but doesn't display the messages. Or, it opens and then won't open the message, it shows the "Check Internet connection...".

Sending using iMail returns something about a NULL to or from the server.

No problems with my Internet, with other online sites nor, any other apps.