Microsoft To-Do Logon Fails on Web

I am logged on fine on the apps, but wanted to check something on the To-Do web app but can't get past the logon screen. I am logged on perfectly to Outlook.com on the web, OneDrive on the web, OneNote on the web, etc. but when I change to To-Do it goes back to the choose account screen!

I have even gone to In Private to complete a fresh logon, and it also bounces back to the logon screen. The service status screen shows no problems, but it looks like there is a problem with the service, at least on the Web app!

Anyone else seen this?