result of a script (external check) executed on a proxy is not reported to the server

result (standard output) of an external check (script) executed on a proxy in passive mode is not reported to the zabbix server.
Result:
Wireshark shows that the script is correctly executed.
Proxy_history table in proxy database contains the output (value field)
Expected:
value field of proxy_history table is reported to zabbix server and appears in "Last Data".