This typically means that the key pairs that have been exchanged between controllers in your High Availability Pair have become invalid for some reason.
To fix this, we briefly make both controllers "Standalone" (making note of which IPs were used to build HA on each side, which controller is configured as "Primary", and which (if any or both) synchronization options have been configured FIRST) and what the Fast Failover timeout value has been set for ... for a minute or two ... thereby breaking the pairing ... and then after that brief stint as two separate and standalone controllers ... re-pair them using the settings you had in place previously.
This should fix things for you.
