I am having the same issue not being able to connect to the server. I tested our server and got this:
Status: Resolving address of ftp.expressbill.com
Status: Connecting to 170.138.220.80
Warning: The entered address does not resolve to an IPv6 address.
<<<<<<<<<HOW DO WE CORRECT THIS WARNING??***********
Status: Connected, waiting for welcome message...
Reply: 220 EFT Server 6.5.16.3
Command: CLNT
https://ftptest.net on behalf of 71.78.184.210
Reply: 200 Noted.
Command: AUTH TLS
Reply: 234 AUTH Command OK. Initializing SSL connection.
Status: Performing TLS handshake...
Status: TLS handshake successful, verifying certificate...
Status: Received 1 certificates from server.
Status: cert[0]: subject='serialNumber=5014375,jurisdictionOfIncorporationCountryName=US,jurisdictionOfIncorporationStateOrProvinceName=Delaware,businessCategory=Private Organization,C=US,postalCode=37214,ST=Tennessee,L=Nashville,street=3055 Lebanon Pike,O=Change Healthcare\5c, Inc.,OU=Web Operations,OU=COMODO EV SSL,CN=ftp.expressbill.com' issuer='C=GB,ST=Greater Manchester,L=Salford,O=COMODO CA Limited,CN=COMODO RSA Extended Validation Secure Server CA'
Command: USER FEC44499
Reply: 331 Password required for fec44499.
Command: PASS ********
Reply: 230 Login OK. Proceed.
Command: SYST
Reply: 215 UNIX Type: L8
Command: FEAT
Reply: 211-Features supported:
Reply: COMB target;source_list
Reply: REST STREAM
Reply: SIZE
Reply: MDTM
Reply: MFMT
Reply: XCRC filename;start;end
Reply: SSCN
Reply: MLST Size*;Modify*;Create;Type*;Unique;Perm*;Lang;Media-Type;CharSet;
Reply: XNOP
Reply: UTF8
Reply: UTF-8
Reply: LANG EN*
Reply: CLNT
Reply: 211 END
Command: PBSZ 0
Reply: 200 PBSZ Command OK. Protection buffer size set to 0.
Command: PROT P
Reply: 200 PROT Command OK. Using Private data connection
Command: PWD
Reply: 257 "/" is current folder.
Status: Current path is /
Command: TYPE I
Reply: 200 Type set to I.
Command: PASV
Reply: 227 Entering Passive Mode (170,138,220,80,112,71).
Command: MLSD
Status: Data connection established, performing TLS handshake...
Reply: 150 Opening BINARY mode data connection for MLSD /.
Status: TLS handshake successful, verifying certificate...
Status: Received 1 certificates from server.
Status: cert[0]: subject='serialNumber=5014375,jurisdictionOfIncorporationCountryName=US,jurisdictionOfIncorporationStateOrProvinceName=Delaware,businessCategory=Private Organization,C=US,postalCode=37214,ST=Tennessee,L=Nashville,street=3055 Lebanon Pike,O=Change Healthcare\5c, Inc.,OU=Web Operations,OU=COMODO EV SSL,CN=ftp.expressbill.com' issuer='C=GB,ST=Greater Manchester,L=Salford,O=COMODO CA Limited,CN=COMODO RSA Extended Validation Secure Server CA'
Status: TLS session of transfer connection has been resumed.
Listing: Type=pdir;Modify=20190829200349;Perm=cdel; ..
Listing: Type=dir;Modify=20170901174234;Perm=cdefl; reports
Reply: 226 Transfer complete. 170 bytes transferred. 170 bps.
Status: Success