A contact is attempting to transfer registration for an event to another contact in CiviCRM version 7.x-5.12.3. It's an old installation that has been upgraded numerous times.
They receive the following error:
DB Error: constraint violation
In the logs we have the following information:
[db_error: message="DB Error: constraint violation" code=-3 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_participant (event_id , status_id , role_id , register_date , source , is_test , is_pay_later ) VALUES ( 232 , 1 , '4' , 20200203 , 'Our Event' , 0 , 0 ) [nativecode=1452 ** Cannot add or update a child row: a foreign key constraint fails (`von_civicrm_backup`.`civicrm_participant`, CONSTRAINT `FK_civicrm_participant_contact_id` FOREIGN KEY (`contact_id`) REFERENCES `civicrm_contact` (`id`) ON DELETE CASCADE)]"]
I'm not sure how to fix this.