How do I resolve a 503 Server Unavailable connection error on VMware ESXi?

I have a VMware ESXi server which has been up and running for nearly 200 days. The past few days, when I try to connect to it using the VMware vSphere Client, I am unable to connect. After entering my username and password, I see the little spinning wheel and the status line say "Connecting..." and then "Loading Inventory...", and then I receive an error message:

The server 'my.host.name' could not interpret the client's request. (The remote server returned an error: (503) Server Unavailable

Call "ServiceInstance.RetrieveContent" for object "ServiceInstance" on Server "my.host.name" failed.

I am able to SSH in to the VMware ESXi server. All virtual machines seem to be up and running fine, so I want to know beforehand if I need to bring them down for maintenance!!! If methods proposed in your answer would interfere with running VMs please clearly state that so I know to prepare for downtime. Thanks!

How can I resolve this error condition on VMware ESXi?

(I'd post the version number but I don't know how to get it without the vSphere console!)

EDIT: Roughly a month after I asked this question, the server inexplicably rebooted itself. I don't know if it panicked or what happened... but after it rebooted, this problem was gone. So I can't test / confirm any of the answer unless the problem reappears (which I hope it does not!)

You need to restart the vmware management services. Luckily, this is easy (since you have SSH access) and non-impactful to the virtual machines.

In a nutshell, SSH to the esx frame as root and then run one of the two following commands (depending on if it is esx/i):

For ESX:

service mgmt-vmware restart

For ESXi:

/sbin/services.sh restart

See this article for full details

I Solved the issue by uninstalling widecap ServiceInstance.RetrieveContent Error