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;
If the scratch partition (/tmp) gets filled up, check for the presence of the file ql_ima.log. If it is present, you are bothered by a bug from an older version of the qla4xxx driver. Even if you use Update Manager, this driver does not get updated automatically. The solution is add the driver to Update Manager or download and install the update manually.
This procedure describes the manual installation of the update on your ESX host. There's no need to put the host in maintenance mode but you will have to reboot to use the new driver.
Download the driver (you need a VMware account):
Unpack the download and copy the file qla4xxx-6220.127.116.11-offline_bundle-1682448.z ...