I created a support ticket which was answered by Mendix. Here's the response:
We've seen similar errors when customer ran into 'file' access issues. In that case, you could add -Djavax.xml.accessExternalSchema=all
to your Java argument (add it to the "Extra JVM parameters" in your server configuration), which should also allow accessing over HTTP (you could try to test locally whether this indeed solves the issue). However, this will not work (to add the JVM variable) on the Mendix Cloud due to security best practices.
Alas it didn't work for me right away and since we plan to utilize the Mendix Cloud anyway this soltion won't suffice.
Just wanted t owrap this question up
best regards
Stefan