I had a similar problem when running... and I had to change the following from this.
C:\>dtexec /SQL \MultipleVersionsTest /Set \package.variables[ConfigConn].Value;
Data Source=DB1;Initial Catalog=Testing;Provider=SQLNCLI.1;Integrated
Security=SSPI;Auto Translate=False;
to this to get it to work.
C:\>dtexec /SQL...