Strangely enough, the revision log entry for 10.5.7 says "Bug fixes: FTP has been updated to the latest version of the OpenSSL libraries, to fix the "heartbleed" security hole."
Still not sufficiently up-to-date though, same problem (slightly different log entry).
Opening Connection home434367050.1and1-data.host:22
Server version: SSH-2.0-OpenSSH_6.7p1 Debian-5+deb8u1~ui80+7
We claim version: SSH-2.0-PuTTY-FZ-Local: Apr 12 2014 12:11:00
Using SSH protocol version 2
Couldn't agree a client-to-server cipher (available: aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com)
SSH: Fatal: Couldn't agree a client-to-server cipher (available: aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com)
Connection closed
I'll have to think about paying for 11 or 12, like I said I only use a very small part of what DOpus has to offer. Until then I'll have to put up with FileZilla.