Intercompany invoicing flow - error on image document class

Hi All,

We have setup intercompany invoices using EDI message setup; here is my setup : 

- Two companies: ERF and Y01 for example (association number similar to company ID in both cases 

- Create an internal customer Y01 and an internal supplier ERF

- The association number of company ERF and internal supplier ERF must be the same 

- The association number of company Y01 and internal customer Y01 must be the same 

- Customer and supplier type is internal 

- Customer Y01 is linked to a company that can supply the internal site (ERF) 

- Supplier ERF is linked to a company that can buy from the internal site (Y01)

- Message configuration = okay for customer and supplier

- On installation site, delivery address is equal to company address  + media message, message class, and receiver setup done

- Our ID at supplier/customer update on both customer and supplier

- Supplier message default + customer message configuration set

- External supplier invoice parameters set for the SUPPEXT 

- Routing address and rule setup done

I then created instant invoice for customer Y01 on company ERF and then tried to send it, i get the following error message in the process : 

NB : the flow of invoicing uses instant invoicing and not manual customer invoice with parts.

Can you please share some ideas on possible issues causing this error ? 

Thank you,

Tanya