I had a small problem on the MacBook Pro. I had just brought it up to date with MacOS updates, using downloaded install packages instead of Software Update, and needed updating Gatekeeper and MRT security data files. I opened LockRattler and every time I clicked the button to List all pending updates it finally said I couldn't reach Apple's update servers.
I checked on my desktop system, which could reach these servers fine. I then tried to click Install all pending updates button instead, but it took no pleasure either – again there was no answer. I then tried Trigger background check which also drew a plot, although I knew that both Gatekeeper and MRT were still outdated.
That's when I noticed TripMode's menu bar icon. I had reviewed this amazing product for Mac | Life and MacFormat, and by completing my review you had turned TripMode off and terminated it. Somehow in the MacOS update process, it had been activated and set there and blocked the outgoing connections that would have given me these updates.
Before trying to use LockRattler to list or install pending updates, make sure that a firewall that blocks outgoing connections is either disabled or will allow these connections. This not only applies to TripMode, which limits data transfer to ensure that you remain within the mobile connection budget, but also to Little Snitch and Patrick Wardle's amazing open source Lulu firewall.