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Backup PM864A randomly goes to fault

We have a redudant pair of PM864A controllers with BC810 interconnection units, the backup CPU randomly goes to fault (Fault LED blinking, Run OFF, Power, Battery and Dual ON), sometimes immediately after startup, sometimes after several hours. Controller's log at the time of the fault:

E 2016-10-19 14:49:05.161 NOERROR != SendResult 1100b, 69643
E 2016-10-19 14:49:05.167 4008 P:RCUMessHalt Lower CPU
W 2016-10-19 14:49:05.162 Unit= _SWTargets Controller_DCS 4002 P:DualTest Error 00000003
E 2016-10-19 14:49:05.165 Unit= _SWTargets Controller_DCS 4008 P:RCUMessHalt Lower CPU
E 2016-10-19 14:49:05.161 Unit= _SWTargets Controller_DCS 4023 B:Bkp RCUMessHalt in Standby

Full controllers' log files are attached for your reference.

Any ideas about the cause of the fault? Thank you.