lftp stucks on ftps connection while uploading a zero byte file

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Environment

SUSE Linux Enterprise Server 12 Service Pack 3 (SLES 12 SP3)

Situation

When uploading a zero byte files using lftp via ftps secured connections the process stucks. Uploading regular files shows no problem.

Resolution

When experiencing this issue, please contact SUSE Technical Services for a Program Temporary Fix.

Cause

A problem in the SSL handshake.

Additional Information

lftp -d -u ftpuser -e 'set ftp:ssl-force true; set ssl:verify-certificate no' ftps://<ftp-server>
[...]
lftp ftpuser@vsftpd.suma3.lan:~/in> put test.txt.0
---> PROT C
<--- 200 PROT now Clear.
---> TYPE I
<--- 200 Switching to Binary mode.
---> EPSV
<--- 229 Entering Extended Passive Mode (|||30008|).
---- Connecting data socket to (192.168.66.180) port 30008
---- Data connection established
---> ALLO 4
<--- 202 ALLO command ignored.
---> STOR test.txt.0
<--- 522 Data connections must be encrypted.
---- Closing data socket
---> PROT P
<--- 200 PROT now Private.
---> EPSV
<--- 229 Entering Extended Passive Mode (|||30032|).
---- Connecting data socket to (192.168.66.180) port 30032
---- Data connection established
---> STOR test.txt.0
<--- 150 Ok to send data.
Certificate verification: common name: â€˜vsftpd.suma3.lanâ€™ matched
---- Closing data socket
<--- 226 Transfer complete.
---> SITE UTIME 20180202150623 test.txt.0
<--- 500 Unknown SITE command.
---> SITE UTIME test.txt.0 20180202150623 20180202150623 20180202150623 UTC
<--- 500 Unknown SITE command.
4 bytes transferred

test.txt.0 was a file having content -> upload works

upload a zero byte file:

lftp ftpuser@vsftpd.suma3.lan:~/in> put test.txt.1
---> EPSV
<--- 229 Entering Extended Passive Mode (|||30100|).
---- Connecting data socket to (192.168.66.180) port 30100
---- Data connection established
---> STOR test.txt.1
<--- 150 Ok to send data.
`test.txt.1' at 0 (0%) [Sending data/TLS]

-> hangs

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