Building r/3.x shows the following traces:
[WARNING] Some problems were encountered while building the effective model for org.opencastproject.assemblies:opencast-karaf-featuresom:3.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-install-plugin is missing. @ org.opencastproject.assemblies:opencast-karaf-features:[unknown-version], /opencast/builds/opencast/assemblies/karaf-features/pom.xml, line 73, column 15
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.