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It could be that some of the services did not start properly on the ESXi or a failure on one of them is happening. If your VMs are properly running on that ESXi and pingable then not much to worry as this is only management error. If you connect to the ESXi over SSH and run:
services.sh restart
Give it a few seconds to finish and let's see if it comes up.
If it fails, run the next commands:
1. /etc/init.d/esxui status
2. tail -n 500 /var/log/vpxa.log
3. tail -n 500 /var/log/hostd.log
4. df -h
5. vdf -h
Copy all these outputs in a text file so we see if there are some errors.