If we allow maven to decide the version number for a release (which generally is what we want to do) then we get the following exception invariably across all our projects:
ERROR: Processing failed due to a bug in the code. Please report this to jenkinsci-users@googlegroups.com
java.lang.NullPointerException
at org.jvnet.hudson.plugins.m2release.M2ReleaseBuildWrapper.getReleaseVersion(M2ReleaseBuildWrapper.java:336)
at org.jvnet.hudson.plugins.m2release.M2ReleaseBuildWrapper.setUp(M2ReleaseBuildWrapper.java:159)
at hudson.maven.MavenModuleSetBuild$RunnerImpl.doRun(MavenModuleSetBuild.java:607)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:429)
at hudson.model.Run.run(Run.java:1375)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:467)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)

The workaround is to click on the radio button "Specify one version for all modules". The default values there are fine.