This means you compiled a class with the wrong java version (49.0 is Java 1.5, I think) and it seems in this case you try to run it in an older Java-VM...
Try to set the javac to compile for an older Version e.g. 1.4 and retry.
You can set this in your build files - assuming ant it would look like this:
<javac srcdir="${src.dir}" destdir="${bin.dir}" debug="on" deprecation="off" optimize="off" includes="**" source="1.4" target="1.4" fork="yes" memoryMaximumSize="256m" encoding="utf-8">
<classpath refid="build.classpath" />
</javac>
Hope this helps,
Cheers, Arndt