When I try to use the Add school/work account feature in Windows 10 to add my Office 365 Small Business Premium account in Windows 10, I get this error message instead: "Something went wrong. We weren't able to register your device and add your account to windows". This happens when adding the account to the Outlook Mail app, when signing in to the Office Mobile apps or when going through the Settings app. When adding the account to the Mail, you first get the error, then a message that the account was added succesfully. However, the account is only added to the Mail app, not system-wide (it is not visible in Settings). In the other cases, it fails completely and the account is not added at all.
The computers are running Windows 10 Pro (one is running Home). I am not using Azure AD - all users on the computers are signed in with Microsoft accounts. I have also tested with the latest Windows 10 Insider Preview with the exact same result.