Hello everybody,
So I dived a bit more in this issue and found that the issue is coming from the connection with the SAP Code List for Product.
I can Lookup the data in the Cloud Solution for Account ID and Ship-to ID, but the Lookup is sending me an error back when I try with the Product ID.
Moreover, if I try to save the Sales Order without any item, I can create a new sales order from the excel template without any problem. However, as soon as I write a Product ID for a line (which is a consistent one), it sends me back the Error 500. So I am not able to create any sales order with items in it.
If anyone has already encounter this, some help would be greatly appreciated. 
Anyway, I will also post an incident to SAP as even the standard form is giving me this error, in 4 different systems.
Thank you for your attention.
Best regards.
Jacques-Antoine