Root cause: in my case it was a discrepancy between the client (who pushes tasks) and the worker ( who received the tasks). In my case the client was an Ubuntu server with Python 3.8, while the client was Debian 9, with Python 3.5.

How to solve: standardize client and worker. I put my client to be also Debian 9 with Python 3.5.

Initial problem message:

worker1 | [2020-05-11 13:10:54,366: WARNING/MainProcess] Received and deleted unknown message. Wrong destination?!?
worker1 |
worker1 | The full contents of the message body was: body: [[4, 4], {}, {‘callbacks’: None, ‘chord’: None, ‘errbacks’: None, ‘chain’: None}] (81b)
worker1 | {content_type:’application/json’ content_encoding:’utf-8′
worker1 | delivery_info:{‘routing_key’: ‘celery’, ‘redelivered’: False, ‘delivery_tag’: 1, ‘exchange’: ”, ‘consumer_tag’: ‘None2’} headers={‘expires’: None, ‘origin’: ‘gen101022@lenovo’, ‘retries’:
0, ‘shadow’: None, ‘root_id’: ‘3ad250be-952c-4dcc-ac3c-31245d83f216’, ‘eta’: None, ‘group’: None, ‘parent_id’: None, ‘task’: ‘tasks.add’, ‘lang’: ‘py’, ‘id’: ‘3ad250be-952c-4dcc-ac3c-31245d83f216’, ‘timeli
mit’: [None, None], ‘kwargsrepr’: ‘{}’, ‘argsrepr’: ‘(4, 4)’}}