The error (Error When Creating Intersoft Shipment: E3333 Error deserializing XML message. Check your XML complies with the API documentation) occurs when the shipment data cannot be processed correctly by the Intersoft API.
To resolve this issue, review the following probable causes:
Check your Royal Mail account setup
Verify your Royal Mail account has been set up in Intersoft.
If not configured, review the Royal Mail Integration guide.
Check your warehouse address
Click Warehouse then Manage Warehouses.
Find the relevant warehouse then click Edit.
Ensure all address fields are completed.
Remove any foreign characters (such as Ε, Γ , Γ«) from the address.
Check courier service codes
Go to Shipping Management then Extras.
Click Courier Service Codes.
Search for the affected service.
Check that ExtraCode6 is empty.
By following these steps, you should be able to resolve the error.