I have migrated my site from 220.127.116.11 to 2.1.4. Almost all of the migrated products are back-ordered, that means the quantity is below 0.
The new 2.1.4 site is configured to support back orders as it is described in the documentation: Store - Configuration - Catalog - Inventory - Product Stock Options - Backorders -> 'Allow Qty Below 0'.
When I try to edit a product that is back-ordered (i.e. Qty < 0) it is impossible I to save it. There is a error message below the Qty box: 'Please enter a valid number in this field.'