Found the problem to be compiler JDK version.
Referring to
http://wiki.jboss.org/wiki/Wiki.jsp?page=DeterminingClassVersionFromADotC...
I found the bytecode major version to be jdk 1.5
Compiled back with 1.4 and the problem is gone.
Strange though, that the error was not java.lang.UnsupportedClassVersionError
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