Hi Carsten,
Yes, it is right that you can't change the customs document of the SO anymore.
This is actually a wrong scenario from business point of view. -> If the SO is blocked by SPL check, why does the user create delivery?
I think this should be a wrong operation by mistake. If so, I would suggest implementing note 589257 and 444204. After that, it will not be possible to create DN for a blocked SO any longer.
Hope the information is helpful.
Best regards,
Hualin