SQL 2000 server upgraded to full version of 2008 R2.
Windows authentication as a member of domain admins
Regardless of the database name...
backup database mydatabase
to disk = 'c:\data\backup\db.bak'
Msg 911, Level 16, State 11, Line 1
Database 'mydatabase' does not exist. Make sure that the name is entered correctly.
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.'
Same with "backup log [any-database] to disk..."
All say "database does not exist".
I've added my user explicitly as dbo.
"use [any-name]" works just fine.