Internal server error caused by Caused by: org.xml.sax.SAXParseException: schema_reference

I have a published web service which takes a string as input and returns a list of objects. When calling the web service (both from a Mendix application and from SOAP UI) I get the following error. Question is how I can debug the cause. I have not been able to find the cause yet. Caused by: org.xml.sax.SAXParseException: schema_reference.4: Failed to read schema document '', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>. EDIT: I have done some more testing and this appears to happen with the simplest of published web services. It is either a bug in 5.9.1 or I am missing a very basic point.
1 answers

We are experiencing the same issue. However it seems that in our cases this is caused by an connection time-out. This makes me wonder does Mendix require an outgoing connection when one fires a webservice request to Mendix. Could someone of Mendix perhaps answer this question?

Edit: We are working in Mendix 5.11.0