I came across this error today, when using the HTML5 client and VMware Update Manager (VUM);
Response with status: 401 OK for URL: https://<FQDN of VUM server>ui/vum-ui/rest/vcobjects/urn:vmomi:HostSystem:host-10:478c8cfc-c88e-4fdb-9e1a-93d899697bf7/isUpdateSupported
Turns out this is something that only affects Firefox since 6.7 U1 and VMware have a KB article on it here; https://kb.vmware.com/s/article/59696
UPDATE – VMware have now fixed this in 6.7 U2 of vCenter which is available from Vmware Downloads
While I was trying to update an iLO from version 2.00 to 2.27 on a HP DL360 G6, I was seeing the firmware update fail in Windows, the error back was that the hardware installed in the server was incorrect. Installing the firmware through the iLO web page itself just failed, appearing to timeout when attempting to upload the image.
The fix for this, oddly enough was to use Firefox. When doing the same update to the iLO in Firefox, the image does get uploaded and the firmware update then completed without any problems.