I am not sure if it is relevant but on the Internal Customer - Address - Customer own ID does it match the “Customers own address ID” on the Company record, ensure that it is the correct address ID for that site is selected. It is not mandatory for Inter-Site transactions to work, but if not present the internal CO created on the supply site will be created as a single occurrence delivery address which may be causing the issue.
On the Supplier Side - on the internal supplier, Address, Suppliers own address ID should be the same as the Document address on the Sales & Procurement site record for that internal supplier
@SVGJOHNSULL looking at the error message, it seems there’s some custom event that is being fired and that event produces the error which is propagated in the Incoming CO.
When is the error happening? When you try to approve the incoming CO? If yes, then maybe there’s an event on inserts in the the Customer Order table and the error in the event blocks the process.
I would suggest to have a look at the events you have on customer_order_tab.