When it says that there was an "Internal server error", there should be a stacktrace in the console or server log. It would be nice to have that one with the corresponding line number as well. Also, Are you purposely listening for "Event"? Becase if it's not called from somewhere outside, this will not be called for 2 reasons: 1. @EventHandler is missing 2. "Event" will not be triggered. No matter what's happening I've never used Expressions before, so I can't tell you, if there's anything wrong.
Numerous things: - Your fields are not initialized - You do should not listen to the super type "Event" - You do not have the @interface annotation for EventHandler - Unless you wrote your own generic "Expression" class, there is no java class for it. If you wrote one, then provide it. What are you actually expecting this code to do...?