This backend version is not supported to design database diagrams or tables. (MS Visual Database Tools)

You may receive the below error and couple of other errors while working with SQL Server 2012

This backend version is not supported to design database diagrams or tables. (MS Visual Database Tools)
Index was outside the bounds of array (Microsoft.SQLServer.SMO)

Workaround

The errors may happen because you are trying to access SQL Server 2012 databases from SQL Server 2008 or SQL Server 2008 R2 management tools. To fix the issue you need to download SQL 2012 management tools.