Please look at these forum topics where other people saw the same error message: Httperror code 303
and Get HTTPError() code:303 when I try to sync
Do you live in a geographic region where internet connections are known to be blocked or otherwise not reliable? Or, are you using a firewall or VPN which might affect your internet connectivity?
Also, you mentioned that your Anki desktop app syncs correctly with “AnkiHub”. However, you did not say whether it syncs correctly with AnkiWeb. Does it?