Vagrant: "Unable to connect" on toolshed installation

I'm testing the Vagrant box of planemo as describe at http://planemo.readthedocs.org/en/latest/appliance.html#launching-the-appliance-vagrant and startup works fine for me. However, when I try to install something from any toolshed through the Galaxy Admin GUI, it get an "Unable to connect" error. This happens once I click on the "Install to Galaxy" button in the tool repository.

I suspect it has something to do with the ports and port forwarding through the host machine, but I don't know how the toolshed tries to reach the Galaxy instance at that point, so no idea how to fix it.