From this error, we can deduce that there is an issue with the dates or date range on the relationship or history between the two organizations. Go to the Organization history tab and check if there are duplicate entries or incomplete/invalid dates. If there are duplicates, edit the entry(ies) to backdate the relationship history and try deleting one of the duplicate lines. Otherwise, make sure the date ranges on that tab are valid and don't conflict with other relationships, past or present.