What is the return type of the microflow that you publish? Try changing it to an object type.
Did you know that Mendix 7.10 allows you to publish REST services without the REST module?
What can be abstracted is that there apparently is a MetaObject that is NULL
at RestServices.StartPublishServices (JavaAction : 'StartPublishesServicesJava')
at REST.REST_AfterStartup (SubMicroflow : 'StartPublishServices')
Maybe this piece of code helps.
It helps us to help you if you would share more details about the analysis that you have done so far. When it happens, when it does not happen. Share the flow, show the JavaAction. Things like that.