Adding an error to slave_transaction_retry_errors cannot be done while the server is running, it has to be added to the my.cnf file and then restart the server. Please make this a dynamic variable. Or at least dynamic enough that it is picked up when the sql_thread (re)starts.

Getting a new network related failure (see MDEV-16543) that is solved by simply restarting the replication slave sql_thread, it was not possible to add it to slave_transaction_retry_errors without restarting the server, which seems a bit unnecessary.