Assume that you receive an email message that contains a custom form and the custom form has an attachment in it. When you drag and drop the attachment to your desktop or other email items, the mouse cursor shows the forbidden sign and you can't drop it.
To completely fix this issue, install February 5, 2019, update for Outlook 2016 (KB4462147) and February 5, 2019, update for Word 2016 (KB4462145) together. Changes and fixes are made in only Outlook 2016. But you may encounter unexpected issues without KB4462145.