I just noticed two issues:
Most of the package mirrors seem outdated, e.g. they do not provide melodic packages. I could resolve this issue by updating the puppet configuration:
sudo puppet apply ros_mirror.pp --modulepath=/etc/puppet/modules:/usr/share/puppet/modules:.
The mirror status of most mirrors is 2018-05-12 on http://wiki.ros.org/Mirrors. This seems to be an upstream problem. The rsync source
rsync.osuosl.org::ros_wiki_mirrorseems to be out of date. Tully, can you fix this?