Hi tanja,
Please check if there is an entry present in the table CRMM_BUT_CRMOBJ for the guid of the business partner. The table stores the site ID of the system where partner is present. Delete the entry from the table and try adding the relationship again.
Please note that there is no maintenance available for the above table. You may have to use a custom program to delete the entry from it.
Hope this helps,
~Athreya