Problem fixed. Apparently maven resolved the plugin prefix `webstart` to the alpha1 version. The search algorithm looks in group org.codehaus.mojo but not in org.codehaus.mojo.webstart.
I fixed this by adding the group in my settings.xml:
<settings>
...
<pluginGroups>
<pluginGroup>org.codehaus.mojo.webstart</pluginGroup>
</pluginGroups>
<settings>
