Ssl – Debugging curl FTPS connection, using Client Authentication certificates (OpenSSL)

(Moved from StackOverflow) I've been asked to setup an FTPS connection, using a Client Authentication certificate. I'm the client, connecting to the server. I'm on a Windows 7 workstation, behind a firewall, although this connection will eventually be used on a dedicated CentOS VM. Some other info:

• TLS 1.2 only
• Passive mode only
• My WAN IP is whitelisted
• I'm connecting to a test URL, for now
• There is no outbound firewall (only inbound rules)
• Key should be sent in PEM format

I bought the certificate from Comodo, and generated the certificate with IE10. I exported the complete certificate from IE, with private key and extended properties, in the .pfx format.

I wrote a little batch file to extract the necessary parts from the .pfx, using OpenSSL:

::Extract the private key from the PFX
openssl pkcs12 -in comodo.pfx -nocerts -out encrypted.key
::Extract unencrypted private key
openssl rsa -in encrypted.key -out decrypted.key
::Extract the public cert in Base64 from the PFX
openssl pkcs12 -in comodo.pfx -clcerts -nokeys | openssl x509 -out comodo.cer
::Extract the chain bundle from the PFX
openssl pkcs12 -in comodo.pfx -nodes -nokeys -cacerts | openssl x509 -out comodo.crt

Using the extracted key parts, I have been trying various curl (v7.46.0) commands to connect to the FTPS server. The connection keeps failing. My latest attempt is below. The error is almost always the same.

curl -3 -k -v --ftp-ssl --tlsv1.2 --ftp-ssl-reqd --ftp-pasv --key decrypted.key --cacert comodo.crt --cert comodo.cer --user REMOVED:REMOVED ftp://ftps.REMOVED/

An example of the output is:

> curl -3 -v -k --user REMOVED:REMOVED --ftp-ssl --tlsv1.2 --ftp-ssl-reqd --ftp-skip-pasv-ip --cert comodo.cer --key priv.pem ftp://ftps.REMOVED/
*   Trying REMOVED_IP.
* Connected to ftps.REMOVED (REMOVED_IP) port 21 (#0)
< 220 Service ready for new user.
> AUTH SSL
< 234 Command AUTH okay; starting SSL connection.
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* Unknown SSL protocol error in connection to ftps.REMOVED:21
* Closing connection 0 curl: (35) Unknown SSL protocol error in connection to ftps.REMOVED:21
>pause
Press any key to continue . . .

Strangely, if I connect with FileZilla, i can seem to get past the TLS handshake, but then I get different errors when I try to upload a file. As this connection will ultimately be made with curl, I've concentrated on this approach.

I've been trying this for over a week. The server admin has bought a certificate from my provider, and can connect without issue. I'm stumped on this one. Any help would be greatly appreciated. General curl / ftps debugging tips would also be appreciated.