very strange problem with To-Do what makes it useless.
I've got "Unable to sync" information on Windows app, Android app and, what is really strange, also in web application (it's under right-top question mark icon). I used it since few months and it works great. Few days ago stopped.
Removed all data on all systems, logout everywhere and delete apps. It changes nothing. After fresh installations problem still exists. Even on different devices. Deleted also all tasks in old Outlook tasks, changed password... nothing helps.
When I added second account in To-Do Windows app it works ok. Mine account isn't.
Technical support after few email didn't give me any advice how to solve it - just some links how can I restore lost tasks (it's not problem - I want sync back again).