Security Enhancements have been introduced in 5.0u6.
See also Release Notes at http://java.sun.com/j2se/1.5.0/ReleaseNotes.html

"Prior to this update, an applet or application could specify the version of the JRE on which it would run. This has changed. All applets are executed with the latest JRE version."

However, the HtmlConverter is still unaware of the new policy.
The HtmlConverter still allows to convert to tags to use only JRE version 1.5.0_07 for example:

http://java.sun.com/j2se/1.5.0/docs/guide/plugin/developer_guide/html_converter_more.html

It is necessary to modify the HtmlConverter, so it will work according to the new SSV policy.