cannot process json request: Can't open JSON object or arr

[kfadia]

hi

zabbix-server.log contains this
18092:20170606:125509.969 cannot process json request: Can't open JSON object or array "null,"clock":1496753709}"

its been repeating, how to debug to get more information about this issue ? what this error mean?

zabbix-server ver. 3.0.9 on ubuntu 14.04 in AWS\
most agents are working in active mode. all of them are above version 3.0

have more than 500 agents with auto-registration.