There was one product in the master data, which was not enabled for re-seller.
So as a first step the website "Reseller" needs to be assigned to all product types (configurable and virtual products).
Then we needed to set all booking with this product manually to NEW in the database (they had status ERROR). Afterwards we checked, if the cron job did pick up all booking in status new and transferred them correctly to Magento.
UPDATE `jfb_b2bcashdesk_order_item` SET `transfer_status`='new' WHERE `sku`='ADDON.RESERVATION.INFOSYSTEM.GEX' and `transfer_status`='error' UPDATE `jfb_b2bcashdesk_order` o JOIN `jfb_b2bcashdesk_order_item` oi ON oi.order_id = o.id SET o.`transfer_status`='new' WHERE o.`transfer_status`='error'