When updating the Universal Plugin Manager, the "Downloading and Installing" dialog never completes.
However, when the user refreshes the page or opens a new tab to Manage Add-ons page, it shows that is was successfully installed.
The UPM update has actually succeeded, and refreshing the page will confirm this (either by displaying the new version string in the footer, or by no longer showing the "UPM update available" notification banner). This is a one-time, front-end bug that customers may encounter now, but will not encounter in the future.
See workaround. Once version 2.16.2 is installed future updates should not encounter the error.