Apply filter on GET /beta/users/<user_id>/chats/getAllMessages fails

Hello I have a call at the endpoint:

/beta/users/<user_id>/chats/getAllMessages?$filter="lastModifiedDateTime gt 2021-07-18T00:00:00.000Z"

with the below code:

url = '%sbeta/users/%s/chats/getAllMessages?$filter="lastModifiedDateTime gt %sT00:00:00.000Z"' % (ms_api_url, user_id, before7days.strftime("%Y-%m-%d")) 
response = requests.get(url,
            headers = {
                'Authorization': 'Bearer %s' % access_token
            }).json()

but I am geting the below error:

{
  'error': {
    'code': 'BadRequest',
    'message': 'Invalid filter clause',
    'innerError': {
      'date': '2021-07-25T09:04:49',
      'request-id': '590c724f-e161-44de-a8d9-a011dec40b6e',
      'client-request-id': '590c724f-e161-44de-a8d9-a011dec40b6e'
    }
  }
}

 what am I doing wrong?