Rene,
There seems to be something wrong with the wsdl. If you upload the wsdl to https://www.wsdl-analyzer.com/upload/uploadWsdl and perform the check you'll get 1 error:
Result: Style Conformity
Priority: ERROR
Description: The binding should conform to the conventions of the used style.
Binding: data_binding
The part of the message 'notify' references a schema type instead of a schema element.
The part of the message 'notify_response' references a schema type instead of a schema element.
The issue thus seems to be related to the wsdl rather than it being a mendix issue. You could try and do some more research on the provided error.