List<String> buildFlavours = ['ubuntu', 'macosx']
List<MyBuildStep> modulesGroups = [
  new MyBuildStep('UnitTestGroup1'),
  new MyBuildStep('UnitTestGroup2')
]

println "*** outside loop, buildFlavours.size() = ${buildFlavours.size()}"
buildFlavours.each { flavour ->
  println "*** inside loop, flavour = ${flavour}"
  println "*** outside loop, modulesGroups.size() = ${modulesGroups.size()}"
  modulesGroups.each { group ->

        // Doesn't work, silently terminates the iteration of the loop with no error either!
        println "*** inside loop, group = ${group}"

        // Somehow this works:
        // String groupCopy = group.toString()
        // println "*** inside loop, group = ${groupCopy}"
    }
}

class MyBuildStep implements Serializable {
    private String displayName

    MyBuildStep(String displayName) {
        this.displayName = displayName
    }

    @Override
    String toString() {
        return displayName
    }
}