This error prevents the mail-ext plugin from being usable.

FATAL: org/sonatype/aether/util/StringUtils
java.lang.NoClassDefFoundError: org/sonatype/aether/util/StringUtils
at hudson.plugins.emailext.plugins.content.ChangesSinceLastBuildContent.evaluate(ChangesSinceLastBuildContent.java:56)
at org.jenkinsci.plugins.tokenmacro.DataBoundTokenMacro.evaluate(DataBoundTokenMacro.java:177)
at org.jenkinsci.plugins.tokenmacro.TokenMacro.expand(TokenMacro.java:182)
at org.jenkinsci.plugins.tokenmacro.TokenMacro.expandAll(TokenMacro.java:233)
at hudson.plugins.emailext.plugins.ContentBuilder.transformText(ContentBuilder.java:63)
at hudson.plugins.emailext.ExtendedEmailPublisher.getContent(ExtendedEmailPublisher.java:705)
at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:488)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:319)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:311)
at hudson.plugins.emailext.ExtendedEmailPublisher.prebuild(ExtendedEmailPublisher.java:261)
at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:838)
at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:833)
at hudson.model.Build$BuildExecution.doRun(Build.java:142)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:586)
at hudson.model.Run.execute(Run.java:1597)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:247)

I suspect that the developer accidently used the wrong StringUtils class. I think he should have used the Apache commons-lang class instead.