It appears you have a custom security module implemented as a servlet
filter. Or, more accurately, the web.xml file refers to one. But that class doesn't appear to be present in your WAR.
It's likely that someone had installed it in one of the Tomcat5 library directories and it wasn't copied into the Tomcat 6 library directory.
I wouldn't expect that there would be a problem if you did that, but one of the major differences between Tomcat6 and Tomcat6 is the elimination of the 3-library setup in favor of a single library, so I'd check the vendor documentation to see if there are any issues.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.