initialization error before compiling

I'm trying to build the excellent Bruce B. Blackshaw Java FTP aplication under JB5 and have an very uncommon error :

"FTPClient.java": Error #: 750 : initialization error: com.borland.compiler.symtab.LoadError: class file has wrong version 48.0

the errors occurs before compiling and helps it JB5 documentation is quite minimal on 750 error msg.

Does somebody knows that ?

[406 byte] By [olliezzz] at [2007-9-26 19:45:11]