Well, SAP could help and the issue is resolved.
In case of anyone else having problems:
First of all, this import feature should not be used, to create order groups.
After export of order group we replaced one character (W) with 2 characters (pw). That was the reason for the failing import, because in every line of the text-document where the replace took replace one blank character had to be removed. After removing this character the import worked.