I don't remember well because it was long time ago when we were running our 1500D clusters with 5.2.x and experienced similar issue. It happens only to slave/passive units and we dealt with Fortinet TAC quite some time and they eventually put a patch on the OS.
I recommend you open a case with Fortinet TAC. It's probably specific to your HA setting.
It looks like there was an issue with the 1500D in FOS v5.2 with a crashing daemon process. Customer support had a patch but the issue resurfaced in v5.4, 5.4.1, 5.6 and 5.6.1 according to this thread: https://forum.fortinet.com/tm.aspx?m=118706
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