could you pelase show errors from server log?
try to delete jsp-api-2.1.jar
thanks for your effort,
but I guess found the the very reason for the problem. The bug has been caused by eclipse loading-troubles. The automatic deployment of the application didn't work perfectly, so some beans hasn't been upgraded. After not just cleaning the project-build but also closing the project and reopening it, the bug disappeard as fast as it had arrived,