I found key for my issue. It is DYNPRO_SEND_IN_BACKGROUND error.
Alos I found old interesting thread regarding this DYNPRO_SEND_IN_BACKGROUND
When I'm trying to add new item to Sales Order system is trying to call dialog/window, but this is not allowed without SAPGUI. I assume this dialog is "reason code" widow I have to enter when I'm trying to save my sales order:
As per investigation I provided above, most RFCs support flags for "dark" processing without user interaction (and creating visual "objects").
Does BAPI_SALESORDER_CHANGE FM has such flag?
Also what is NOBINPT='X parameter? Where can I set it?
Are there other ways how to avoid dialog calling?