Any folder with Hide Collaborators checked off in Folder Settings.
You may have been able to do this in the past, but this is a recent security update. Therefore, this is expected behavior.
You will need to uncheck "hide collaborators" if you would like to use Microsoft Online to edit documents.
The issue is that Microsoft online shows the names names/faces of the other editors, which should not be done when Hide Collaborators is checked off. Currently Box Notes prevents collaborative editing in this case, but O365 does not and therefore must be blocked when using this setting.