Solution:
1)include non-free packages
2)install sun-java5-jre (this really should have been all that was required but no... one more step...)
3)edit opera preferences:
tools, preferences, advanced tab, content section, Java Options button, add the path /usr/lib/jvm/java-1.5.0-sun-1.5.0.10/jre/lib/i386/ and validate with the validate button.
4)restart opera the error messages should be gone.