This error might occur if mailbox disks fails. Mailbox disks are used to store data related to the HA-pair that needs to be persistent across reboots. This data includes information about the cluster state, state of the mirrors, whether a shutdown was performed in a ‘clean’ way or not and so on. For more details on how to troubleshoot mailbox disk errors you can refer KB article, https://kb.netapp.com/support/index?page=content&id=1010888
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