Kalender wird nicht angezeigt

Es gibt eine Adresse, die nicht abgerufen werden kann. Deswegen hat dieser Kalender gar keine Termine gerade.
Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.7/site-packages/icalendar/parser.py", line 340, in parts
    validate_token(name)
  File "/app/.heroku/python/lib/python3.7/site-packages/icalendar/parser.py", line 128, in validate_token
    raise ValueError(name)
ValueError: <!doctype html>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/app/.heroku/python/lib/python3.7/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/app/app.py", line 229, in get_calendar
    entries = get_events(specification)
  File "/app/app.py", line 209, in get_events
    for events in events_list:
  File "/app/.heroku/python/lib/python3.7/concurrent/futures/_base.py", line 586, in result_iterator
    yield fs.pop().result()
  File "/app/.heroku/python/lib/python3.7/concurrent/futures/_base.py", line 425, in result
    return self.__get_result()
  File "/app/.heroku/python/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/app/.heroku/python/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/app/app.py", line 208, in <lambda>
    events_list = e.map(lambda url: retrieve_calendar(url, specification), urls)
  File "/app/app.py", line 159, in retrieve_calendar
    calendars = icalendar.Calendar.from_ical(calendar_text, multiple=True)
  File "/app/.heroku/python/lib/python3.7/site-packages/icalendar/cal.py", line 330, in from_ical
    name, params, vals = line.parts()
  File "/app/.heroku/python/lib/python3.7/site-packages/icalendar/parser.py", line 354, in parts
    % (self, exc)
ValueError: Content line could not be parsed into parts: '<!doctype html>': <!doctype html>
Brandenburger-Maker / calendar-federation

Kalender wird nicht angezeigt #9
