Maybe it's a clue?

Yes, investigate what happens in your network every day at that time!!!

Good evening Salva, I am seeing the same problem. Every 50GB or so the SFTP connection will drop, retrys are not successful. scp an sftp work correctly, even in parallel to this activity so it looks like it is a Net::Foreign issue. Environment:

matt@aslan:185% cat /etc/redhat-release 
Fedora release 22 (Twenty Two)
matt@aslan:186% perl -v
This is perl 5, version 20, subversion 3 (v5.20.3) built for x86_64-li
+nux-thread-multi
(with 16 registered patches, see perl -V for more detail)
...
matt@aslan:187% perl_module_version Net::SFTP::Foreign
Net::SFTP::Foreign  : 1.75
matt@aslan:188%
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I see no relevant detail in the ssh logs during the failure:

...
debug3: Ignored env DISPLAY
debug3: Ignored env SGI_ABI
debug1: Sending subsystem: sftp
debug2: channel 0: request subsystem confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: subsystem request accepted on channel 0
# got it!, len:95, code:2, id:-, status: -
          Error: Connection lost (Connection to remote server is broke
+n)
            Resuming transfer (retry 1)
# queueing msg len: 71, code:17, id:1973 ... [1]
# waiting for message... [1]
# queueing msg len: 83, code:3, id:1974 ... [2]
# waiting for message... [2]
debug2: channel 0: read<=0 rfd 4 len 0                            
debug2: channel 0: read failed
debug2: channel 0: close_read
debug2: channel 0: input open -> drain
...
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The error and resuming lines above was generated by this perl, the rest was from -vvv:

        emit("          Error: " . $connect->status . ' (' .
            $connect->error . ')',
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emit() is just a wrapper around print STDERR; return undef; The code whole paragraph is:

    for (my $pass=0; $pass < $_max_retries; $pass++) {
        if ($connect->put($local, $remote, %$options)) {
            my $stat = $connect->stat($remote);
            $size = $stat->size;
            main::add2Log("        Finished $size of $lsize bytes", 'i
+nfo')
                if ($main::debug >= 1 and $pass);
            $main::progressBar = 100; 
            return 1;
        }    
        main::append2Log("\b"x$_fll, ' 'x$_fll, "\b"x$_fll)
            unless (main::is_graphical());
        emit("          Error: " . $connect->status . ' (' .
            $connect->error . ')', 
            "            Resuming transfer (retry " . ($pass+1) . ')')
+;
        my $connect = sshConnect($url, $user, $password, 'sftp', undef
+, 1,
            1);  
        return undef unless ($connect);
    }
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The whole run sans debug looks like:

      Uploading marilyn/photos/1108BlueRidge/Original/CRW_2030.tif    
+       
          Error: Connection lost (Connection to remote server is broke
+n)     
            Resuming transfer (retry 1)
          Error: Connection lost (Connection to remote server is broke
+n)     
            Resuming transfer (retry 2)
          Error: Connection lost (Connection to remote server is broke
+n)     
            Resuming transfer (retry 3)
          Error: Connection lost (Connection to remote server is broke
+n)     
            Resuming transfer (retry 4)
        Retries exceeded - failed to send '/Website/www/html/marilyn/p
+hotos/1108BlueRidge/Original/CRW_2030.tif'
      Uploading marilyn/photos/1108BlueRidge/Original/CRW_2031.crw
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What detail can I collect to help diagnose? Matt