Hi Martin,
1. Please check in Tcode SMW3_00 ( customizing: SPRO->SAP Customizing Implementation Guide->Customer Relationship Management->CRM Middleware and Related Components->Message Flow Setup->BDoc Type Settings) if there is an entry BUS_TRANS_MSG with flag "Do not Send".
If so, please uncheck the flag "Do not send", or delete the entry.
2. Please check the inbound scheduler in tcode SMQR, whether there are queues unregisterd for business transactions.
Best regards,
Ellen