It seems that the cert file that can be downloaded from the fix api page has expired and does not actually match what is currently used on api.bitfloor.com.
You can get current cert file with following command:

openssl s_client -showcerts -connect api.bitfloor.com:5252

From the output copy the lines starting from -----BEGIN CERTIFICATE----- up to and including -----END CERTIFICATE----- and save those to the .crt file
that is in stunnel configuration.