If you receive an automated email from Re-Leased with the subject "Billing Contact for Tenancy was Archived" it means the contact associated with the tenancy specified in the email has been archived.
This could have happened by either archiving or merging the contact in Xero. If the contact was merged there will be a current version of them available to connect the Re-Leased tenancy to.
To correct it, open the tenancy mentioned in the message, then go to Tenancy Details on the left menu. Clear out the name against the Xero Invoice contact and re-type it so that it matches the current version of this contact.