After installing RTM on my laptop, I all of my Beta 2 Linq to SQL diagrams refused to open and save the error.
“Cannot load ‘path’: Error encountered, check Error List window for details”.
The error list had the following error
“Error 2 There is no Unicode byte order mark. Cannot switch to Unicode. 1 1 TestingLinqToSQL”
In the properties for the dialog, it was already set as Unicode. To fix this, in the xml header for the diagram, in Beta 2 it was ““. RTM requires the encoding to be utf-8 (). Set the xml declaration to this, save and reopen and your diagram should be working again.