I have been able to reproduce this issue by installing ca-certificates (20120212) and ca-certificates-java (20111223) on squeeze and upgrading to ca-certificates (20120623) and ca-certificates-java (20120721).
Between ca-certificates-java 20111223 and 20120721 the packaging of the UpdateCertificates class changed from a raw .class file in /usr/share/ca-certificates-java/ to a jar files. On upgrading ca-certificates and ca-certificates-java simultaneously the old trigger of ca-certificates-java is invoked and fails to run the .class file now replaced with a jar. The trigger mechanism has been reworked in ca-certificates-java 20121112+nmu1, and upgrading to this version doesn't generate a NoClassDefFoundError. I also verified that this error doesn't appear when upgrading from squeeze to wheezy. Emmanuel Bourg __ This is the maintainer address of Debian's Java team <http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-java-maintainers>. Please use debian-j...@lists.debian.org for discussions and questions.