Google Cal: Error 400

Google Calendar stopped working for me, with “Error response:400”

This is in the log:

Logger: homeassistant.components.calendar
Source: components/google/ 
Integration: Calendar (documentation, issues) 
First occurred: 6:03:11 PM (6 occurrences) 
Last logged: 6:03:11 PM

google: Error on device update!
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/", line 358, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/usr/src/homeassistant/homeassistant/helpers/", line 466, in async_device_update
    await task
  File "/usr/local/lib/python3.8/concurrent/futures/", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google/", line 106, in update
  File "/usr/src/homeassistant/homeassistant/util/", line 285, in wrapper
    result = method(*args, **kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google/", line 169, in update
    service, params = self._prepare_query()
  File "/usr/src/homeassistant/homeassistant/components/google/", line 132, in _prepare_query
    service = self.calendar_service.get()
  File "/usr/src/homeassistant/homeassistant/components/google/", line 347, in get
    http = credentials.authorize(httplib2.Http())
AttributeError: 'NoneType' object has no attribute 'authorize'

How to reauthorize? Thanks!