ESXi host Failing to connect to endpoint.

I have Vmware ESXi that I recently upgraded to 6.5 build 7388607 and on a lot of my hosts I've been getting this error:

503 Service Unavailable (Failed to connect to endpoint: [N7Vmacore4Http16LocalServiceSpecE:0x0465c900] _serverNamespace = / action = Allow _port = 8309)

A lot of what I am finding is this error occurring on vC\enter.  I do have vCenter however it has been running fine.  When this error occurs, of course vCenter can't see the host but that's not the issue.  What's causing the error in the first place?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
did you update vCenter ?
Appears you are connecting via vsphere client c# client. have you tried with webcleint
huntsonAuthor Commented:
All of these connections have been with the web client - all through a web GUI.
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