When the 2nd HA instance connects to a STANDALONE HA instance, it can happen that the replication channel is closed with the message "heartbeat counter reaches 0".
Heartbeat is started right after the connection of the replication channel has been established. During the subsequent synchronization phase, the STANDALONE instance turns into ACTICE_SYNC state. In this state AND if the Replicated File Store is used, a copy of the persistent store is created. This can take longer than the so-called "heartbeat missing threshold" and thus, the connection is closed.
If you use the Replicated File Store and if your store is quite large (more than 200 MB), it is recommended to increase the "Heart Beat Missing Threshold" (attribute name is "heartbeat-missing-threshold") of the HA Controller Swiftlet. E.g. double it from the default 10 to 20.
Unfortunately there is a bug in releases prior to 7.0.0. The connector of the replication channel stops connecting after the replication channel has been closed because of "heartbeat counter reaches 0". Solution is here to delete and recreate the connector of the replication channel. The bug has been fixed for 7.0.0.