OpenSSH 6.9 version have disabled obsolete form of KEX "Diffie-Hellman group exchange" algorithm.
One solution is to move "Diffie-Hellman group exchange" KEX to last position in "Algorithm selection policy" setting in Putty. Since FileZilla's SFTP support is based on PuTTY's psftp. If we change the default key exchange cipher priorities in PuTTY, it should also affect FileZilla. That's very easy to access in Windows: http://vogelchr.blogspot.com.es/2015/07/public-service-announcement-putty.html In Linux, one posible temporal fix it is to look how to access to this preferences. Another posible temporal fix it's to see if it is posible to change from Filezilla key auth to system ssh user agent auth But it seems that the definitive fix it's the update to filezilla 3.13.1 version: 3.13.1 (2015-08-24) changelog - Fix SFTP component crashing on some combinations of encryption and key exchange algorithm
