Version 2.4 of the maven-clover-plugin uses Clover 1.3.13, which doesn't recognise new Atlassian-issued Clover licences.
You need to use version 2.4.1 of the plugin, which is hosted at http://repository.atlassian.com/maven2.
You'll need to update your pom.xml with the new version:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-clover-plugin</artifactId>
<version>2.4.1</version>
<configuration>
<licenseLocation>...your licence file path...</licenseLocation>
...
</configuration>
</plugin>
and add the Atlassian public repository as a plugin repository in your pom.xml or ~/.m2/settings.xml file:
<pluginRepositories>
<pluginRepository>
<id>atlassian-m2-repository</id>
<url>http://repository.atlassian.com/maven2</url>
<snapshots>
<enabled>false</enabled>
</snapshots>
</pluginRepository>
</pluginRepositories>