Any folder with Hide collaborators and their activity from non-owners selected 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 clear the Hide collaborators and their activity from non-owners setting i the privacy section of Folder Settings 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 and their activity from non-owners 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.