As Beate says, have a look in the database XXX and see what owner you've got. If the owner is yyy, you have to change that to xxx.
Read more in the following OSS notes
- 551915 - R/3 won't start after a database copy
(conversion scripts attached to this note, if STM doesn't work)
- 683447 - SAP Tools for MS SQL Server (STM)
- 151603 - Copying an SQL Server Database
the schema conversion option in the SAP Tools is supposed to be used for databases where the owner of the tables and the other objects is currently 'dbo'.
If this is not the case then you will receive this error message.
Take a look at the tables in the XXX database, do they really belong to the user dbo?
If not, then there is no need for a schema conversion.