Why does `grunt watch` fail with "ENOENT, no such file or directory" although file exists

When I run grunt watch, watch works once: Grunt compiles/minifies all files, after I've edited and saved a Javascript file. But when I edit and save that same file again, Grunt says ENOENT, no such file or directory 'client/debiki/debiki-action-edit.js'.

(The file mentioned is the file I just modified.) However the file does exist:

$ ll client/debiki/debiki-action-edit.js
-rw-rw-r-- 1 kajmagnus kajmagnus 22497 Jan 21 23:47 client/debiki/debiki-action-edit.js

And if I run grunt directly from the command line (without watch) Grunt is always able to compile everything without any error.

What's happening? How can I make grunt watch work?

Details:

Ubuntu Linux, Grunt v0.3.17, Node.js v0.8.4. I use the built-in watch task, not grunt-contrib-watch.

The complete Grunt error message

Running "watch" task
Waiting...
/usr/local/lib/node_modules/grunt/lib/grunt/file.js:75
      throw grunt.task.taskError(e.message, e);
                       ^
TaskError: ENOENT, no such file or directory 'client/debiki/debiki-action-edit.js'
    at Task.taskError (/usr/local/lib/node_modules/grunt/lib/util/task.js:59:17)
    at /usr/local/lib/node_modules/grunt/lib/grunt/file.js:75:24
    at Array.filter (native)
    at Object.expandByType (/usr/local/lib/node_modules/grunt/lib/grunt/file.js:69:40)
    at Timer.<anonymous> (/usr/local/lib/node_modules/grunt/tasks/watch.js:158:44)
    at Timer.exports.setInterval.timer.ontimeout (timers.js:234:14)