Output of the "cphaprob state" command shows that one of the cluster members is in "Down" state.
Output of the "cphaprob list" command on the problematic member shows: Device Name: Synchronization
Current state: problem
However - and this is very important distinction, at the same time it shows: Device Name: Filter
Current state: ok
Output of "fw ctl pstat" command on both members shows in the "Sync:" section that there is a huge discrepancy
(or even zero values) in the "Sync packets sent:" subsection - in the "total:" counter values.
Status: Able to Send/Receive sync packets
Sync packets sent: total : <VALUE>, ...
$FWDIR/log/fwd.elg file on cluster member might repeatedly show: fwsyncn_connected: connection to IP_address_of_peer_member failed. error=-N
(example: fwsyncn_connected: connection to 192.168.2.7 failed. error=-5).
Most probably, the Synchronization (sync) mechanism was not fully initialized (after 'cpstop;cpstart' command or after reboot)