Could not resolve class java $ lang $ Enum

Question

Okay, so that puzzled me, and the only answer I got from the person who wrote the code in the first place was “yes, it happens sometimes” ...

We have an application written entirely using the Groovy framework. (The application is a Play! Plugin, but I think it does not matter.) Unfortunately, it is very rare that when we start the system and send requests, we get the following error:

file:/path_to_file/Language.groovy: 6: unable to resolve class java$lang$Enum 
 @ line 6, column 1.
   public enum Language {
   ^

file:/path_to_file/Language.groovy: -1: unable to resolve class java$lang$Enum 
     @ line -1, column -1.
2 errors

As you can see, Language.groovy is just an enumeration; it defines some enum values, some static helper methods for common operations, etc. Why do I get "unable to resolve the java class $ lang $ Enum class and only rarely?

Unfortunately, Google could not enlighten me regarding a possible source. I am ashamed to admit that I even turned to Bing for help with the expected results. Any help would be greatly appreciated. Thank!