Hi @Deyan_Deyanov ,
I think that the issue is that your Redshift cluster in account B is not able to read data in your S3 bucket in account A.
RDB loader connects to Redshift to run the COPY statement but then Redshift needs to access the data on s3.
You might want to take a look at this page :
https://docs.aws.amazon.com/redshift/latest/dg/copy-usage_notes-access-permissions.html