Nexus staging plugin error (probably its version is too old)

[Aklakan]

master$ mvn -P release clean install deploy

results in:

[ERROR] Failed to execute goal org.sonatype.plugins:nexus-staging-maven-plugin:1.6.8:deploy (injected-nexus-deploy) on project rmltk-bom: Execution injected-nexus-deploy of goal org.sonatype.plugins:nexus-staging-maven-plugin:1.6.8:deploy failed: An API incompatibility was encountered while executing org.sonatype.plugins:nexus-staging-maven-plugin:1.6.8:deploy: java.lang.ExceptionInInitializerError: null