Freezes somewhere inside Ruby on generate

[altavir]

I've got generate task configured like this:

task generate(type: JavaExec, dependsOn: ":site:gather") {
    group 'grain'
    logging.captureStandardOutput LogLevel.INFO
    classpath sourceSets.main.runtimeClasspath
    main = mainClassName
    args["generate"]
    doFirst {
        println "Starting generate"
    }
}

When I try to run it, gradle freezes on generate task without even displaying Starting generate. VisualVM shows that execution freezes somewhere inside JRuby thread selector. When I try to force use newer version of JRuby: compile 'org.jruby:jruby:9.1.10.0', I get the error: Exception in thread "Thread-22" java.lang.NoSuchMethodError: jnr.constants.platform.OpenFlags.defined()Z. So it is probably JRuby or ruby script bug.