I recently upgraded our vCenter environment to version 6.7 Update 1 and started noticing that Update Manager related tasks and windows were not displaying and instead a red banner with 401 response error kept coming up. Unfortunately I was unable to find any resolution when the issue first came up, but more recently I found that it only manifests itself if you use Firefox as your browser of choice, which is in fact the scenario I've been facing.
VMware suggested workarounds:
clear your Firefox browser cache
use a different browser.
A fix should be upcoming, until then, those are your possible solutions. Source: VMware KB59696