Hello Christian,
As other SCN members have mentioned, kindly refresh the IDoc metadata and then try posting the data -
For Single Stack Systems (Java Only) -
Managing IDoc Metadata Using IDoc Metadata Monitor - Advanced Adapter Engine - SAP Library
http://saphelp.me/idoc-metadata-sap-pi-single-stack/
For Dual Stack systems (ABAP + Java) -
1. Login to IDX2 T-code from your SAP PI system
2. Choose the system where your Z-IDoc is imported.
3. Delete the entry of IDoc
4. Re-import the IDoc by providing all the details.
Let me know if this will fix the issue.
Regards,
Nitin Deshpande