You do not go into great detail on your configuration, but I have sucesfully copied Alfresco around between servers before doing the following:-
1. Shutdown alfresco completely.
2. Backup alf_data using tar
3. Copy the tar to the new server
4. Make sure Alfresco is completely shutdown.
5. Delete the existing alf_data
6. Restore my tar file
7. Start Alfresco
This does rely on the fact that you have a default install where all the data is in alf_data if your install differs then the above procedure would need modifying.
From your error message it looks as if there are pointers in postgres that do not exist in the content store which infers that the postgres database backup was taken at a later time than the content store backup. I guess it is also just possible this could be a permissions problem on the content store, (although I would have expected a 403 error for that). Does user id running Alfresco have the necessary permissions on alf_data?