Did you resolve this one? I am having this issue at the moment after a database recovery process.

*Some*, not all tables need to have a db user specified, while others don't

in SQL Server Enterprise Manager the query has to be

   select * from [xyz_dbo].[some_table_name]

On others

   select * from [some_table_name]

Works just fine

???

I take it to be some default setting on the SQL Server to default the db user name in the absence of one being supplied... but I know not about such things.