I raised https://github.com/ben-manes/gradle-versions-plugin/issues/16 for it.
Execution failed for task ':dependencyUpdates'. > java.lang.ClassNotFoundException: org.gradle.api.internal.artifacts.version.LatestVersionSemanticComparator
This is more or less an FYI report for your “Known issues” section.
I think the ‘dependencyUpdates’ functionality should be integrated into Gradle. It’s very helpful but also quite fragile since it depends on Gradle internals.