You didn't post the results of the query I posted, but I suspect the user doesn't exist in the target database, or is orphaned.
USE db1
CREATE USER DeleteMyRows FROM LOGIN DeleteMyRows
or
USE db1
ALTER USER DeleteMyRows WITH LOGIN = DeleteMyRows
I've never tried creating a stored procedure to do operations in more than one database at a time. Have a read about cross-database ownership chaining and see what works for you. Don't enable CDOC if you can avoid it - it's a security minefield!
John