I know it's too late.but ..hey
the problem is resolved.
I first worked with my report iReport3.7.1 then I used the iReport3.7.2:
that's why I added the JAR JR3.7.2 and I deleted the JAR JR3.7.1 my project, but it is not removed from the directory / build / WEB-INF/lib.
I found the two JARs in this directory: this is the problem.
This works for me too. We had two versions of the Jasper's JARs in the classpath