If your ESX host has lost connection to the vCenter server, but you can still directly access the host's management, and can find nothing else wrong with the vCenter server or its network connection to the host, you may want to the free space on your ESX host.

Logs and other temporary files may have filled up one or more ramdisk partitions (e.g. when you don't have a persistent scratch partition, it gets mounted on /tmp). This can cause any of the following issues:

Connection loss between the ESX host and vCenter server;

Errors when attempting to reconnect or add an ESX host to a vCenter server;

Errors while attempting to activate or configure HA;

Other unexplainable errors...

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