I would like to bring your attention to https://github.com/geonetwork/core-geonetwork/pull/4595
The repository repo.boundlessgeo.com is being shutdown, and we have setup repo.osgeo.org to act as a replacement. We also took the opportunity to replace the webdav folder used by the GeoTools project.
For background see news announcement, and a trick with settings.xml to work on older projects.
While we will put a redirect in place not all build tools are able to follow a redirect, and we encourage all GeoNetwork active branches (and forks) to update.
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Jody Garnett