I've installed the locked-file report plugin and when I deliberately lock a file and trigger a build I get:

Searching for locked files in workspace.
FATAL: hudson.matrix.MatrixConfiguration cannot be cast to hudson.model.TopLevelItem
java.lang.ClassCastException: hudson.matrix.MatrixConfiguration cannot be cast to hudson.model.TopLevelItem
at hudson.plugins.lockedfilesreport.LockedFilesReporter.checkForFileUsage(LockedFilesReporter.java:79)
at hudson.plugins.lockedfilesreport.LockedFilesReporter.prebuild(LockedFilesReporter.java:53)
at hudson.model.AbstractBuild$AbstractRunner.preBuild(AbstractBuild.java:675)
at hudson.model.AbstractBuild$AbstractRunner.preBuild(AbstractBuild.java:670)
at hudson.model.AbstractBuild$AbstractRunner.preBuild(AbstractBuild.java:666)
at hudson.model.Build$RunnerImpl.doRun(Build.java:121)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:429)
at hudson.model.Run.run(Run.java:1374)
at hudson.matrix.MatrixRun.run(MatrixRun.java:137)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)