Can't take archive lock while synaptic is running

[errors.ubuntu.com bug bridge]

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 90, in __enter__
    return self._lock.__enter__()
apt_pkg.Error: E:Could not get lock /var/cache/apt/archives/lock. It is held by process 2600 (synaptic)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/unattended-upgrade", line 2512, in <module>
    sys.exit(main(options))
  File "/usr/bin/unattended-upgrade", line 1986, in main
    res = run(options, rootdir, mem_log, logfile_dpkg,
  File "/usr/bin/unattended-upgrade", line 2307, in run
    kernel_pkgs_kept_installed) = do_auto_remove(
  File "/usr/bin/unattended-upgrade", line 1876, in do_auto_remove
    res, error = cache_commit(cache, logfile_dpkg, verbose)
  File "/usr/bin/unattended-upgrade", line 861, in cache_commit
    res = cache.commit(install_progress=iprogress)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 675, in commit
    with self._archive_lock:
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 92, in __enter__
    raise LockFailedException(("Failed to lock directory %s: %s") %
apt.cache.LockFailedException: Failed to lock directory /var/cache/apt/archives/: E:Could not get lock /var/cache/apt/archives/lock. It is held by process 2600 (synaptic)

--

The Ubuntu Error Tracker has been receiving reports about a problem regarding unattended-upgrades. This problem was most recently seen with package version 2.1, the problem page at https://errors.ubuntu.com/problem/4eb313e6525a788c7921331a31ca051b830f1fc6 contains more details, including versions of packages affected, stacktrace or traceback, and individual crash reports.
If you do not have access to the Ubuntu Error Tracker and are a software developer, you can request it at http://forms.canonical.com/reports/.