"Copy to" is deaktivated

Hello Forum,

I have a foreign Addon which creates Sales Invoices. I can cancel the invoice, but I cant use the "Copy to" button on the right lower corner to create a Credit Note. I'm wondering why is the button disabled?

1. I tried to re-construct the invoice manually in the GUI: With the same customer, same articles, same payment terms, same...

After adding, this invoice still has a "Copy to" button. But the other invoice created by the addon looks like the same, but without the button.

2. I tried to deactivate the addon, but the "Copy to" button is still inactive. (In case the addon itself deactivates the button.)

I know that invoices without price numbers dont have the "Copy to" button. But this is not the case. Is there a trick or a hidden property that is not shown in the normal invoice view.

Thanks

Mark

PS: The "Copy to" button is in reality a combobox.