I don't have an office365 account - I have a Hotmail account that I can access through the browser in logging into Microsoft Outlook in which I have no issues.
The problem has occurred with Windows 10 desktop mail in which I previously was fully able to view emails, download and view attachments which now provides a 'download failed' message from around October.
I don't want to start deleting or changing accounts - it all worked when I first upgraded and started using Windows 10 and the desktop mail icon. I will just continue to use it through the browser but very disappointing Microsoft - think the issue lies with you and you need to provide a resolution to it.