Already found an answer that seems to work for Mx.
For every operation in the binding section I added the line " <soap:operation soapAction = <operation name>/>
This solved the problem.
The binding section of the WSDL now looks like this:
<wsdl:binding name="com-Binding" type="esb:com-PortType">
<soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
<wsdl:operation name="OpvoerenFactuur">
<soap:operation soapAction = "OpvoerenFactuur"/>
<wsdl:input>
<!--soap:body message="esb:OpvoerenFactuurVraag" part="body" use="literal"/-->
<soap:body use="literal"/>
</wsdl:input>
<wsdl:output>
<soap:body use="literal"/>
</wsdl:output>
<wsdl:fault name="error">
<soap:fault name="error" use="literal"/>
</wsdl:fault>
</wsdl:operation>
<wsdl:operation name="OpvoerenJournaalpost">
<soap:operation soapAction = "OpvoerenJournaalpost"/>
<wsdl:input>
<soap:body use="literal"/>
</wsdl:input>
<wsdl:output>
<soap:body use="literal"/>
</wsdl:output>
<wsdl:fault name="error">
<soap:fault name="error" use="literal"/>
</wsdl:fault>
</wsdl:operation>
<wsdl:operation name="OpvragenRelatie">
<soap:operation soapAction = "OpvragenRelatie"/>
<wsdl:input>
<soap:body use="literal"/>
</wsdl:input>
<wsdl:output>
<soap:body use="literal"/>
</wsdl:output>
</wsdl:operation>
</wsdl:binding>