I am using Google App Engine along with Grails AppEngine plugin. At some point I started getting an exception when I was trying to start my development server. The only thing I had done before I got the error was- I had switched from App Engine SDK 1.2.5 to 1.2.8.
The problem is that the Grails clean command doesnt do a very good job of cleaning the cache folder when the App engine is installed.
The solution was to simply delete the cache folder created by cache and presto ! all was well again