This error typically occurs when:
1. you create a booking, which causes an invoice to be created in Xero
2. in Xero, you create a batch payment for that invoice.
3. you update the booking, which causes Bookeo to try and update the invoice
Unfortunately Xero does not allow updating authorized or paid invoices that have a batch payment. This is not possible even from within Xero.
In these cases, the Xero integration will keep working, but the particular invoice involved won't be updated.
Payments synced by Bookeo are never batched, so this issue can only occur if you create batch payments from inside Xero control panel.