Playframework 2.0 ClassNotFoundException

I used the framework for developing a web application. During development, I always used "play run" to start the server.

Today I tried to start the server for deployment by running "play start", and then received the error as follows:

java.lang.NoClassDefFoundError: Files
Caused by: java.lang.ClassNotFoundException: Files
        at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: Files.  Program will exit.
Exception in thread "main"

I think this is a "space in the way" because the name "missing class" "Files" reminds me of "Program Files". But my game was installed in a directory without a place. In addition, I could always run "play run".

Is there any other place where I should look?

UPDATE: there is a similar question here, but the solution does not work for me.