Hi SoIP,
Are you referring to the Customer:Job field?
Have you selected an existing customer with the dropdown, or created a new one with the ... button? If you just typed a name into the dropdown but that customer doesn't exist, this would cause that error to occur.
Regarding your second issue, have you modified this screen at all? On my end, the action "Show Screen in Popup" isn't used in the Print Preview button. If you have modified the screen, you could delete that action or test that same button on the stock Sales Order screen. Go to Customize > Screens > search for the original screen (Quickbooks_SalesOrder), then hover your mouse over its name and click View to the left.
The issue regarding the invoice screen seems to indicate that there have been some changes made to the "Show Screen In Popup" action in the Create Invoice button. You'll need to modify that button to investigate that action, the most likely scenario is that the wrong screen is being called. You can also compare against the original screen like I mentioned in the paragraph above. Ensure you select the stock "QuickBooks_SalesOrder_CreateInvoice" screen. If you have a copied version of the screen which has been customized, then you can select that copy.
-Russell