What this looks like is that the server is refusing the OPTS UTF8 ON command because we haven't sent a CLNT client_type request.
A bit of searching indicates that CLNT is a non standard extension to identify a client. Why the FTP server should require it before you can turn UTF8 on, I don't know.
The library we use for connecting to FTP doesn't support the CLNT extension. Apparently FileZilla sends it immediately before turning UTF8 on. Something to think about @ivandeex ?