Servlet forwarding does not work

I've created two servlets: hello, which displays "Hello", and world, which displays "World". They work fine when invoked:

http://localhost:2990/jira/plugins/servlet/hello

displays "Hello" and

http://localhost:2990/jira/plugins/servlet/world

displays "World"

Now, I want to forward the hello to the world servlet when a forward parameter is set to "true":

http://localhost:2990/jira/plugins/servlet/hello?forward=true

Then, JIRA says /jira/plugins/servlet/world not found!!!

Even worst, if the url-pattern of the hello servlet (/hello) is changed to a string with more than 5 characters (the length of world) then an exception is raised.