filezilla – ftp PASV inconsistency

I am using pyftpdlib as a python ftp server and filezilla as a client: some of my colleagues can’t upload any files while I have absolutely no issues.

I checked filezilla debug logs and found one weird behavior I don’t understand (I replaced my public ftp server IP with 216.58.215.227 for security reasons)

Command:    PASV
Trace:  CFtpControlSocket::OnReceive()
Response:   227 Entering passive mode (216.58.215.227,207,87).
Trace:  CFtpRawTransferOpData::ParseResponse() in state 2
Trace:  CControlSocket::SendNextCommand()
Trace:  CFtpRawTransferOpData::Send() in state 4
Trace:  Binding data connection source IP to control connection source IP 10.8.8.15

10.8.8.15 is my vpn address: is it normal I get my vpn ip in this part of the logs ?

stranger, when a colleague can’t upload to the ftp, he gets

Command:    PASV
Trace:  CFtpControlSocket::OnReceive()
Response:   227 Entering passive mode (216.58.215.227,207,87).
Trace:  CFtpRawTransferOpData::ParseResponse() in state 2
Trace:  CControlSocket::SendNextCommand()
Trace:  CFtpRawTransferOpData::Send() in state 4
Trace:  Binding data connection source IP to control connection source IP 127.0.0.1

It seems weird to me that filezilla binds data connection to 127.0.0.1 and this I think it explains why upload fails.

How comes this behavior occurs ?

Furthermore: I don’t understand from where the problem comes from: is it a client/server/network issue ?

FYI: the only config directives I edited on pyftpdlib are :

permit_foreign_addresses: True
use_sendfile: False
ac_in_buffer_size: 1000000
ac_out_buffer_size: 500000