Please modify vsftpd.conf as follows:
1. change xferlog_std_format=YES to xferlog_std_format=NO
2. Add log_ftp_protocol=YES
vsftpd.conf usually resides in the directory /etc/vsftpd
After modify the configuration file
1. Stop vsftpd when no one is using it.
2. Check any orphan vsftpd processes and start it again to make sure vsftpd is cleanly using the new configuration.
3. Do a problematic file transfer as mentioned.
4. In the directory /var/log, read the corresponding vsftpd log entries.
5. If you are not sure how to interpret it, post the corresponding log entries here. If you post it here, you may want to remove user-id, file name, etc. in the entries as they may be sensitive.
Remember to remove the modification in vsftpd.conf after taking log or it may affect your system somehow.