Hi Julia,
Could you please post your "network tab error message" ( with screenshots).
1. Check your payload data. Some times it could be different from GW payload.
Goto segw transaction---> Run your service ---> add payload ---->F8 ( if works)
Check the same data from SAPUI5.
I think here you used new date for DatumHinzu field. It might be a problem
Thanks,
KA