We are using duplicity 0.7.17 and Ubuntu 18.04 LTS.

After upgrading these packages in Ubuntu:

Upgrade: libc-bin:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)
Upgrade: locales:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)
Upgrade: multiarch-support:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)
Upgrade: libc6-dev:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), 
Upgrade: libc6:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)
Upgrade: libc-dev-bin:amd64 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)

the duplicity/lftp support stopped working. It worked for years just fine before that.

Basic error message is:

Giving up after 5 attempts. BackendException: Error running 'lftp -c "source /tmp/duplicity-gVCsqa-tempdir/mkstemp-serY3Q-1; ( cd etc_backup/ && ls ) || ( mkdir -p etc_backup/ && cd etc_backup/ && ls )"': returned -11, with output:

Error/debug output:

Using archive dir: /root/.duplicity/etc_daily
Using backup name: etc_daily
GPG binary is gpg, version 2.2.4
Import of duplicity.backends.acdclibackend Succeeded
Import of duplicity.backends.azurebackend Succeeded
Import of duplicity.backends.b2backend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.dpbxbackend Failed: No module named dropbox
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.hubicbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.lftpbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.mediafirebackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.multibackend Succeeded
Import of duplicity.backends.ncftpbackend Succeeded
Import of duplicity.backends.onedrivebackend Succeeded
Import of duplicity.backends.par2backend Succeeded
Import of duplicity.backends.pydrivebackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.ssh_paramiko_backend Succeeded
Import of duplicity.backends.ssh_pexpect_backend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.sxbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
LFTP version is 4.8.1
Using temporary directory /tmp/duplicity-_Mgype-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-_Mgype-tempdir/mkstemp-55iPLR-1
SETTINGS:
set ssl:verify-certificate true
set ftp:ssl-allow false
set http:use-propfind true
set net:timeout 30
set net:max-retries 5
set ftp:passive-mode on
debug
open -u '<username_removed>,<password_removed>' ftp://hostname.your-storagebox.de

Main action: remove-all-but-n-full
Acquiring lockfile /root/.duplicity/etc_daily/lockfile
================================================================================
duplicity 0.7.17 (February 26, 2018)
Args: /usr/bin/duplicity remove-all-but-n-full 7 -v9 --archive-dir=/root/.duplicity --force --name etc_daily --no-encryption ftp://<username_removed>@<hostname_removed>.your-storagebox.de/etc_full
Linux <hostname_removed> 4.15.0-122-generic #124-Ubuntu SMP Thu Oct 15 13:03:05 UTC 2020 x86_64 x86_64
/usr/bin/python2 2.7.17 (default, Sep 30 2020, 13:38:04)
[GCC 7.5.0]
================================================================================
CMD: lftp -c "source /tmp/duplicity-_Mgype-tempdir/mkstemp-55iPLR-1; ( cd etc_full/ && ls ) || ( mkdir -p etc_full/ && cd etc_full/ && ls )"
Reading results of 'lftp -c "source /tmp/duplicity-_Mgype-tempdir/mkstemp-55iPLR-1; ( cd etc_full/ && ls ) || ( mkdir -p etc_full/ && cd etc_full/ && ls )"'
Backtrace of previous error: Traceback (innermost last):
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 369, in inner_retry
    return fn(self, *args)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 578, in list
    return [tobytes(x) for x in self.backend._list()]
  File "/usr/lib/python2.7/dist-packages/duplicity/backends/lftpbackend.py", line 181, in _list
    _, l, e = self.subprocess_popen(commandline)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 499, in subprocess_popen
    (logstr, result, stdout + '
' + stderr))
 BackendException: Error running 'lftp -c "source /tmp/duplicity-_Mgype-tempdir/mkstemp-55iPLR-1; ( cd etc_full/ && ls ) || ( mkdir -p etc_full/ && cd etc_full/ && ls )"': returned -11, with output:
-rw-r--r--   1 u234538-sub2 u234538    885032 Oct 28 00:00 duplicity-full-signatures.20201028T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    885113 Oct 29 00:00 duplicity-full-signatures.20201029T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886172 Oct 30 00:00 duplicity-full-signatures.20201030T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886175 Oct 31 00:00 duplicity-full-signatures.20201031T000002Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886163 Nov  1 00:00 duplicity-full-signatures.20201101T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886173 Nov  2 00:00 duplicity-full-signatures.20201102T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886625 Nov  3 00:00 duplicity-full-signatures.20201103T000002Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538       481 Oct 28 00:00 duplicity-full.20201028T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64187446 Oct 28 00:00 duplicity-full.20201028T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       480 Oct 29 00:00 duplicity-full.20201029T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64189301 Oct 29 00:00 duplicity-full.20201029T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       480 Oct 30 00:00 duplicity-full.20201030T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198883 Oct 30 00:00 duplicity-full.20201030T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       479 Oct 31 00:00 duplicity-full.20201031T000002Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198893 Oct 31 00:00 duplicity-full.20201031T000002Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       482 Nov  1 00:00 duplicity-full.20201101T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198890 Nov  1 00:00 duplicity-full.20201101T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       479 Nov  2 00:00 duplicity-full.20201102T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198894 Nov  2 00:00 duplicity-full.20201102T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       481 Nov  3 00:00 duplicity-full.20201103T000002Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64203538 Nov  3 00:00 duplicity-full.20201103T000002Z.vol1.difftar.gpg

---- Resolving host address...
---- 2 addresses found: 202:4f8:b23:3000::16, 146.202.112.16
---- Connecting to <hostname_removed> (202:4f8:b23:3000::16) port 21
<--- 220 ProFTPD Server (Backup) [202:4f8:b23:3000::16]
---> FEAT
<--- 211-Features:
<---  AUTH TLS
<---  CCC
<---  CLNT
<---  EPRT
<---  EPSV
<---  HOST
<---  LANG ko-KR.UTF-8;ko-KR;es-ES.UTF-8;es-ES;bg-BG.UTF-8;bg-BG;it-IT.UTF-8;it-IT;zh-CN.UTF-8;zh-CN;ja-JP.UTF-8;ja-JP;zh-TW.UTF-8;zh-TW;ru-RU.UTF-8;ru-RU;fr-FR.UTF-8;fr-FR;en-US.UTF-8;en-US*
<---  MDTM
<---  MFF modify;UNIX.group;UNIX.mode;
<---  MFMT
<---  MLST modify*;perm*;size*;type*;unique*;UNIX.group*;UNIX.groupname*;UNIX.mode*;UNIX.owner*;UNIX.ownername*;
<---  PBSZ
<---  PROT
<---  REST STREAM
<---  SIZE
<---  SSCN
<---  TVFS
<---  UTF8
<--- 211 End
---> CLNT lftp/4.8.1
<--- 200 OK
---> LANG
<--- 200 Using default language en_US
---> OPTS UTF8 ON
<--- 200 UTF8 set to on
---> HOST <hostname_removed>
<--- 504 <hostname_removed>: Unknown hostname provided
---> OPTS MLST modify;perm;size;type;UNIX.group;UNIX.mode;UNIX.owner
<--- 200 MLST OPTS modify;perm;size;type;UNIX.group;UNIX.mode;
---> USER <username_removed>
<--- 331 Password required for <username_removed>
---> PASS <password_removed>
<--- 230 User <username_removed> logged in
---> PWD
<--- 257 "/" is the current directory
---- CWD path to be sent is `/etc_full'
---> CWD /etc_full
<--- 250 CWD command successful
---> EPSV
<--- 229 Entering Extended Passive Mode (|||54612|)
---- Connecting data socket to (202:4f8:b23:3000::16) port 54612
---- Data connection established
---> LIST
<--- 150 Opening ASCII mode data connection for file list
---- Got EOF on data connection
---- Closing data socket
<--- 226 Transfer complete
---> QUIT
<--- 221 Goodbye.
---- Closing control socket


Attempt 1 failed. BackendException: Error running 'lftp -c "source /tmp/duplicity-_Mgype-tempdir/mkstemp-55iPLR-1; ( cd etc_full/ && ls ) || ( mkdir -p etc_full/ && cd etc_full/ && ls )"': returned -11, with output:
-rw-r--r--   1 u234538-sub2 u234538    885032 Oct 28 00:00 duplicity-full-signatures.20201028T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    885113 Oct 29 00:00 duplicity-full-signatures.20201029T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886172 Oct 30 00:00 duplicity-full-signatures.20201030T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886175 Oct 31 00:00 duplicity-full-signatures.20201031T000002Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886163 Nov  1 00:00 duplicity-full-signatures.20201101T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886173 Nov  2 00:00 duplicity-full-signatures.20201102T000001Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538    886625 Nov  3 00:00 duplicity-full-signatures.20201103T000002Z.sigtar.gpg
-rw-r--r--   1 u234538-sub2 u234538       481 Oct 28 00:00 duplicity-full.20201028T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64187446 Oct 28 00:00 duplicity-full.20201028T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       480 Oct 29 00:00 duplicity-full.20201029T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64189301 Oct 29 00:00 duplicity-full.20201029T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       480 Oct 30 00:00 duplicity-full.20201030T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198883 Oct 30 00:00 duplicity-full.20201030T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       479 Oct 31 00:00 duplicity-full.20201031T000002Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198893 Oct 31 00:00 duplicity-full.20201031T000002Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       482 Nov  1 00:00 duplicity-full.20201101T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198890 Nov  1 00:00 duplicity-full.20201101T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       479 Nov  2 00:00 duplicity-full.20201102T000001Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64198894 Nov  2 00:00 duplicity-full.20201102T000001Z.vol1.difftar.gpg
-rw-r--r--   1 u234538-sub2 u234538       481 Nov  3 00:00 duplicity-full.20201103T000002Z.manifest.gpg
-rw-r--r--   1 u234538-sub2 u234538  64203538 Nov  3 00:00 duplicity-full.20201103T000002Z.vol1.difftar.gpg


Using

Duplicity 0.7.17 
Python 2.7.17
Ubuntu 18.04 LTS.