There are no reports of this error either. It seems extremely odd, because InstallScape should be using (and appears to be using) the Java installation that comes with InstallScape. So since the java and the classes are from the same installation, it seems very odd that it should fail in this way.
I'm wondering whether you are getting strange problems due to (say) using ftp to transfer files across from Windows to Linux (and doing so in ASCII mode) so that files are getting corrupted? Or maybe you have some odd file system that is being used. The symptoms you keep hitting seem rather unique - so it's hard to see what is unusual about your scenario. As I said before, contacting customer support is definitely the wisest thing to do here.