FTP socks5 proxy (PuTTY) does throw a proxy error

I just configured the advanced FTP to use the socks5 proxy as provided by PuTTY. Unfortunately DOpus can not connect to any FTP locations since an "Invalid PROXY name or address" error is shown. However the same PuTTY instance can be used by Firefox or other FTP clients to access remote FTP sites over socks5 without any problems.