Below you can find a list of common errors, their causes, and steps to resolve:
Contact #XXX - Contact cannot be empty
Please check if the Contact # referenced has a Name or Company Name set. Sometimes, there may be a space character in one of the name fields causing this error.
Contact #XXX - Another contact has this contact number. Please enter a different contact number.
This error occurs when a previously synced contact has been archived (or merged) in Xero. You will need to unarchive the connected contact (linked contacts can be identified by a "Contact Code" being displayed at the bottom of the contacts profile page in Xero).
Invoice #XXX - Account code must be specified
Please check that you have a 'Default Account Code' set on the module's settings page in WHMCS.
Invoice not of valid status for modification
You should manually update the 'Next Invoice to Sync' number to the first invoice that has not yet been sent to Xero.
WHMCS Payments Sync
Invoice #XXX - Document could not be found
This means that you are attempting to sync a payment for an invoice that is not yet in Xero. You will need to sync/create this invoice or increase the 'Next Transaction to Sync' number to skip over it.
Invoice #XXX - Payment amount exceeds the amount due on this invoice
This occurs when there are payments in WHMCS that amount to more than the outstanding balance of the invoice in Xero. Xero does not currently support overpayments so this would need to be handled manually. You can skip transactions that cause problems from being synced by manually incrementing the 'Next Transaction toto Synchronise' number. You can find a transaction ID by going to 'Billing' -> 'Transaction List' and looking at the id in the URL when viewing a transactions details.