duplicity backup reports an unknown failure during backup

[Cali]

Ubuntu 14.04 (freshly installed).

The backup (continous backup from my 12.04 installation) did not finish correctly
and reported the following unknown error.
I deleted all existing backup files and restarted a fresh backup in another location -> i receive the same error message.

My backup destination is a local FTP server (NAS)

Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1494, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1488, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1337, in main
    do_backup(action)
  File "/usr/bin/duplicity", line 1458, in do_backup
    full_backup(col_stats)
  File "/usr/bin/duplicity", line 542, in full_backup
    globals.backend)
  File "/usr/bin/duplicity", line 424, in write_multivol
    (tdp, dest_filename, vol_num)))
  File "/usr/lib/python2.7/dist-packages/duplicity/asyncscheduler.py", line 145, in schedule_task
    return self.__run_synchronously(fn, params)
  File "/usr/lib/python2.7/dist-packages/duplicity/asyncscheduler.py", line 171, in __run_synchronously
    ret = fn(*params)
  File "/usr/bin/duplicity", line 423, in <lambda>
    async_waiters.append(io_scheduler.schedule_task(lambda tdp, dest_filename, vol_num: put(tdp, dest_filename, vol_num),
  File "/usr/bin/duplicity", line 314, in put
    backend.put(tdp, dest_filename)
  File "/usr/lib/python2.7/dist-packages/duplicity/backends/giobackend.py", line 145, in put
    self.copy_file('put', source_file, target_file)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 321, in iterate
    % (n, e.__class__.__name__, str(e)))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 26: ordinal not in range(128)

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: duplicity 0.6.23-1ubuntu4
ProcVersionSignature: Ubuntu 3.13.0-24.46-generic 3.13.9
Uname: Linux 3.13.0-24-generic x86_64
ApportVersion: 2.14.1-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Apr 29 20:12:59 2014
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-04-18 (11 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
SourcePackage: duplicity
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Cali (7-stefan)]

Instead of the local ftp connection to me NAS,
i've tried a smb connection with success (same NAS, same location).
DéjaDup finished the backup without any errors.