Weblate time-out: The file lock '(...)/index.lock' could not be acquired

Originally created by @zen on #17713 (Redmine)

<code class="text">
2020-05-14 02:06:07,973 INFO tails/install-inc-overview/pt: committing pending changes (pending unit)
2020-05-14 02:08:08,004 ERROR Internal Server Error: /translate/tails/install-inc-overview/pt/
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.5/dist-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.5/dist-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/share/weblate/weblate/trans/views/edit.py", line 486, in translate
    request, translation, this_unit_url, next_unit_url
  File "/usr/local/share/weblate/weblate/utils/ratelimit.py", line105, in rate_wrap
    return function(request, *args, **kwargs)
  File "/usr/local/share/weblate/weblate/trans/views/edit.py", line 291, in handle_translate
    go_next = perform_translation(unit, form, request)
  File "/usr/local/share/weblate/weblate/trans/views/edit.py", line 229, in perform_translation
    form.cleaned_data['state']
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/local/share/weblate/weblate/trans/models/unit.py", line 899, in translate
    propagate=propagate
  File "/usr/local/share/weblate/weblate/trans/models/unit.py", line 543, in save_backend
    self.translation.commit_pending('pending unit', request)
  File "/usr/local/share/weblate/weblate/trans/models/translation.py", line 412, in commit_pending
    with self.component.repository.lock:
  File "/usr/local/lib/python3.5/dist-packages/filelock.py", line 322, in __enter__
    self.acquire()
  File "/usr/local/lib/python3.5/dist-packages/filelock.py", line 258, in acquire
    raise Timeout(self._lock_file)
filelock.Timeout: The file lock '/var/lib/weblate/repositories/vcs/tails/index.lock' could not be acquired.
</code>

High priority because prevents users from using the website to translate.