Interesting issue when using database diagrams. I used one happily on Friday to map the relationships in the Data Warehouse at the table level. I hoped that this will make it easier further down the line.
Today I have found that it will not let me use the diagram as the database has not got a valid owner. In the properties of the database I am listed as the owner. I can only assume that this is because I am not on a network that can validate the domain login. Browsing and changing it to SA and all works fine. Odd though.