I found an issue with Diagram and Schema in SQL-Server 2016 that could be useful to the subject. I was editing diagram (related to, and with lot of tables of, the "sales" schema) and added a new table, BUT forgot to declare it schema, so it was with the default "dbo". Then when I returned to, and opened up, the schema "sales" and tried to add an existent table... Bluf! THAT Triggered exactly the same error described in that thread. I even tried the workaround (drag the table) but it didn't work. Suddenly I noticed that the schema was incorrect, I updated it, tried again, and Eureka! the problem was immediately away... Regards.