However, you should be aware of some incompatibility between a VUM plug-in in vSphere Client and Mozilla Firefox.
Update Manager gives a ‘Response with status: 401 Unauthorized for URL‘ error message at random occasions when used with the recent versions of this web-browser (tested with Firefox 63.x and 64.x):
Searching on the VMware’s web-site led me to the KB 59696 that describes the similar issue in vSphere 6.7 and the workaround provided worked for me.
First you need to clear the browser cache:
Then stop Firefox from caching pages by modifying the browser’s default configuration:
According to VMware, they are currently working towards resolving this issue in future releases.