Does this also happen with other FTP clients from the same machine? Also, are you using SSL, etc? A bit more information would be nice.

Finally I had to use gftp to transfer my files (I also tried on windows on the same machine with smartFTP and it worked fine). With my previous version (lftp-3.4.6-1), I had no troubles.
Please find enclosed my lftp.conf file.
The ftp server is ftpperso.free.fr (free is a very popular ISP in France).

I made a full upgrade of my arch distri 2 days ago...

I also used ncftp to make the tranfer and it worked fine.
I also re-compile sources. I have the same concern...
It seems there's a bug in this version...

I've made my own package from old sources (version 3.2.1 : used in the slackware 10.2). And it works fine with this version...

Hmm... what files are corrupted? plain text or binary?

Plain text files are corrupted (in my case, it was a .php file)

That's because there are two modes for file transfer: binary and ASCII. Sending as ASCII text will convert all line endings (\r\n, \n ,\r) to apropriate for server OS.
All ftp clients have option "auto" that desides which file should be transferred as binary and which as text, depending on file extension. You should modify your config file.

And what is the exact synthax of this option ? I've foun nothing in the man pages

I don't use lftp, but I've just installed it to see by myself.
lftp should send all files in binary mode by default. :-/
get1, mget, mput & put commands have -a option to override this default and transfer in ascii mode.
I suppose you don't append -a to these commands. So it's something else causing the corruption.
Have you checked if binary files are not corrupted (images, for example)?

I've detected the problem with .php files. Please find below one of the guilty files. I've just done an transfer attempt a few minutes ago. Still the same issue...
Tell me if it helps. I have some big doubts... At the time being, I've switched to ncftp as ftp client because it works... But I still prefer lftp (some habits :-))

BTW, what "corrupted" means in this particular case?

lftp> put Hft.php

It displays successively :
Delaying before retry
4721 bytes transferred in 2 seconds (1.9K/s)

And actually, nothing has been uploaded...

Hmm... weird. I don't know what is this. For a "bug" in lftp it looks very unusual. :-/

If I try to upload this same file on another ftp server, I don't have this kind of problem...

I've run lftp in debug mode : Here is the output

lftp www.thiefaine@ftpperso.free.fr:/ACTU> put Hft.php
---> PASV
<--- 227 Entering Passive Mode (212,27,63,3,223,43).
---- Connecting data socket to (212.27.63.3) port 57131
---- Data connection established
---> ALLO 4721
---- Closing data socket
<--- 500 'ALLO': command unrecognized.
---> STOR Hft.php
<--- 425 Can't open data connection.
4721 bytes transferred in 2 seconds (1.9K/s)



lftp www.thiefaine@ftpperso.free.fr:/ACTU> dir
<--- 421 No Transfer Timeout (45 seconds): closing control connection.
---- Closing control socket
---- Connecting to ftpperso.free.fr (212.27.63.3) port 21
<--- 220 Serveur de mise a jour des pages perso de Free.fr version [Jan 30 2006 17:28:04]
---> FEAT
<--- 500 'FEAT': command unrecognized.
---> AUTH TLS
<--- 500 'AUTH': command unrecognized.
---> USER www.thiefaine
<--- 331 Password required for www.thiefaine.
---> PASS XXXX
<--- 230 User www.thiefaine logged in.
---> CWD /ACTU
<--- 250 CWD command successful.
---> PASV
<--- 227 Entering Passive Mode (212,27,63,3,32,154).
---- Connecting data socket to (212.27.63.3) port 8346
---- Data connection established
---> LIST
<--- 150 Opening ASCII mode data connection for file list
---- Got EOF on data connection
---- Closing data socket
drwxr-xr-x 3 web site 4096 Nov 16 18:26 .
drwx------ 21 web site 4096 Oct 16 17:12 ..
<--- 226-Transfer complete.
<--- 226 Quotas: utilisation de 392 Mo sur les 1024 Mo
-rw-r--r-- 1 web site 15246 May 23 2001 ChatHft010521.htm
-rw-r--r-- 1 web site 3090 Jun 1 2001 Edito010529.htm
-rw-r--r-- 1 web site 1295 Oct 4 2005 GalPopup.php
-rw-r--r-- 1 web site 17892 Nov 15 19:40 HftActuMenu.php
-rw-r--r-- 1 web site 10716 Sep 29 2001 HftAlbum01-1.htm
-rw-r--r-- 1 web site 5651 Oct 2 2001 HftPresentation.htm
-rw-r--r-- 1 web site 4776 Sep 19 2003 Machin.php
-rw-r--r-- 1 web site 4517 Jul 21 2003 Machin_020502.php
drwxr-xr-x 2 web site 4096 Sep 20 2005 PHOTOS
-rw-r--r-- 1 web site 7921 Oct 4 2005 Scandale2005.php
-rw-r--r-- 1 web site 9495 Nov 15 19:40 Update.htm

On the other ftp server (successful transfer), I've got this output :

lftp machin@machin.shimpinomori.net:~/public_html/ACTU> put Hft.php
---> TYPE I
<--- 200 Type set to I
---> PASV
<--- 227 Entering Passive Mode (219,127,89,42,215,192).
---- Connecting data socket to (219.127.89.42) port 55232
---- Data connection established
---> ALLO 4721
---- Closing data socket
<--- 202 No storage allocation necessary.
---> STOR Hft.php
<--- 150 Opening BINARY mode data connection for Hft.php
<--- 226 Transfer complete.
4721 bytes transferred in 2 seconds (2.3K/s)




lftp machin@machin.shimpinomori.net:~/public_html/ACTU> dir
---> TYPE A
<--- 200 Type set to A
---> PASV
<--- 227 Entering Passive Mode (219,127,89,42,215,196).
---- Connecting data socket to (219.127.89.42) port 55236
---- Data connection established
---> LIST
<--- 150 Opening ASCII mode data connection for file list
-rwxrwxr-x 1 machin www-data 20656 Sep 30 10:55 ActuMenu.php
---- Got EOF on data connection
---- Closing data socket
-rw-r--r-- 1 machin www-data 1295 Jan 17 2005 GalPopup.php
<--- 226 Transfer complete.
-rw-r--r-- 1 machin www-data 4721 Nov 16 18:32 Hft.php
-rwxrwxr-x 1 machin www-data 3210 Apr 23 2006 Mac.php
-rwxrwxr-x 1 machin www-data 8553 Aug 29 18:09 Update.htm
-rwxrwsr-x 1 machin www-data 43 Dec 12 2004 index.php

See, it's a bug in FTP server. :)
OK, I'm closing this "bug" now.