Login failure

Hi,

We have following setup:
Login name: abc\xxxxxx
User Name: dbo
default schema: dbo
database role member: db_owner

When user try to login get failure message. Cannot login. When I tried to give it datareader permission there is error: "cannot use the special principal 'dbo' (Microsoft SQL Error: 15405). Same setup is working with other databaes except one. Any help. This is SQL Server 2000 database migrated to 2008 r2, still have 2000 compatibility level. Other databaes are also in same format and working fine.

Thanks