When testing user.addmedia with the following data set an empty result is received.

Sent:
{"auth":"6c7163e94be7f39a719a6d3b47165ad9","id":8,"method":"user.addmedia","params":{"{users":[

{"userid":23}

],"medias":

{"severity":"45","period":"\"1-7,00:00-23:59\"}

]","sendto":"[email protected]","active":2}},"jsonrpc":"2.0"}

Result:
{"jsonrpc":"2.0","result":

{"userids":[]}

,"id":8}

Checking the Zabbix UI shows the user in question does not show the media added.

Tested against latest release (1.8.4rc3)