This is often caused by the merging of contacts in Xero.
The contact for the employee that is used by FlexiTime has a contact code associated with it, and it is this number which FlexiTime uses to tell Xero which contact to post the invoice to.
When merging contacts in Xero, none of the to-be-archived contact's details are copied to the remaining contact. Only transactions are moved across. Because Xero will not let FlexiTime post invoices to an archived contact, if the archived contact is the one with the contact code that FlexiTime uses, the posting will fail.
The solution is to restore the archived contact, then re-merge, ensuring the previously archived contact (with the contact code) is the one that is retained.