Unable to connect to http://mextest.inf.elte.hu/updates/mirrors

I was trying to set up a development environment, and full build failed with the following message:
Failed to load p2 metadata repository from location http://mextest.inf.elte.hu/updates/mirrors/plantuml: Unable to connect to repository http://mextest.inf.elte.hu/updates/mirrors/plantuml/content.xml: Connection timed out: connect.

It looks like the repositories are not available.