Okay, debugging helps.
"Problem" is class "SCWM/CL_GOODS_MOVEMENT", method "PREPARE_DOCUMENT_CANCEL".
Here's a check against table /scwm/tdlvppgic "Post Goods Issue for Consumption Posting".
Here's a flag "Cancel GI" - Cancel GI only by Call from External System.
In my case, it's empty (I thought that this means, cancellation is possible from external or internal).
When I set this flag, everything will be fine.
Could someone confirm that this flag needs to be set to cnacel the GI from my ERP system? For me seems not be logical as from the F4 help it's only saying that this flag is used to prevent cancellations within EWM (if set).
Regards
Michael