The remote host is missing an update to proftpd-dfsg announced via advisory DSA 1925-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201925-1

It has been discovered that proftpd-dfsg, a virtual-hosting FTP daemon, does not properly handle a '\0' character in a domain name in the Subject Alternative Name field of an X.509 client certificate, when the dNSNameRequired TLS option is enabled. For the stable distribution (lenny), this problem has been fixed in version 1.3.1-17lenny4. For the oldstable distribution (etch), this problem has been fixed in version 1.3.0-19etch3. Binaries for the amd64 architecture will be released once they are available. For the testing distribution (squeeze) and the unstable distribution (sid), this problem has been fixed in version 1.3.2a-2. We recommend that you upgrade your proftpd-dfsg packages.