Gulp - Error: listen EADDRINUSE :::35729?

Why do I get this error:

[00:10:35] Using gulpfile /var/www/html/xxxxx/gulpfile.js
[00:10:35] Starting 'sass'...
[00:10:36] Finished 'sass' after 307 ms
[00:10:36] Starting 'watch'...
[00:10:36] Finished 'watch' after 435 ms

... Uhoh. Got error listen EADDRINUSE :::35729 ...
Error: listen EADDRINUSE :::35729
    at Object.exports._errnoException (util.js:1050:11)
    at exports._exceptionWithHostPort (util.js:1073:20)
    at Server.setupListenHandle [as _listen2] (net.js:1259:14)
    at listenInCluster (net.js:1307:12)
    at Server.listen (net.js:1406:7)
    at Server.listen (/var/www/html/xxxxx/node_modules/tiny-lr/lib/server.js:138:15)
    at exports (/var/www/html/xxxxx/node_modules/gulp-livereload/gulp-livereload.js:23:14)
    at Gulp.<anonymous> (/var/www/html/xxxxx/gulpfile.js:28:18)
    at module.exports (/var/www/html/xxxxx/node_modules/orchestrator/lib/runTask.js:34:7)
    at Gulp.Orchestrator._runTask (/var/www/html/xxxxx/node_modules/orchestrator/index.js:273:3)

I don't understand it. This is my gulpfile:

 var gulp = require('gulp'),
    autoprefixer = require('gulp-autoprefixer'),
    livereload = require('gulp-livereload'),
    sass = require('gulp-sass');


var paths = {
  scripts: ['static/js/**/*.js'],
  sass: ['static/sass/**/*.scss'],
  sassLibs: ['static/js/libs/sass-bootstrap/lib'],
  cssOutput: 'static/dist/css/',
  templates: ['templates/**/*.*'],
  python: ['src/website/**/*.py'],
};


gulp.task('sass', function () {
    return gulp.src(paths.sass)
        .pipe(sass({
            includePaths: paths.sassLibs
        }))
        .pipe(autoprefixer("last 1 version", "> 1%", "ie 8", "ie 7"))
        .pipe(gulp.dest(paths.cssOutput));
});


gulp.task('watch', ['sass'], function () {
    var server = livereload();

    var reloadPaths = []
        .concat([paths.cssOutput])
        .concat(paths.scripts)
        .concat(paths.templates)
        .concat(paths.python);

    // Recompile SASS files when they have changed.
    gulp.watch(paths.sass, ['sass']);

    // Notify livereload server when files have changed.
    gulp.watch(reloadPaths).on('change', function(file) {
        server.changed(file.path);
    });
});


/*
 * Make a complete build.
 */
gulp.task('default', ['sass'], function () {

});

It is working fine on my dev server but not on my production server.

Any ideas?

via teelou