I've been trying to manually add some old console games I'm emulating so I can track which ones I've finished, but Galaxy has some troubles trying to search for some games. For example, I can't find Earthbound, which has an entry on IGDB, and Galaxy can find games with "Earth" on their titles, but once I add the letter B it just suddenly stops and the "We couldn't load data" popup appears. I've also tried that with "Super Mario Bros.", once I add the Bros to the title, it'll just stop.
I've also tried with another PC with a fresh install, and the problem seems to persist. Is anyone having the same problem?

Not having the exact same problem as you, but am certainly having issues with the manual addition feature.
Months ago, the box art for Wolfenstein: Blade of Agony suddenly disappeared for me and hasn´t reappeared since (even though it is on IGDB).
And just today I noticed that Hearts of Iron II: Iron Cross, which I also added manually, has disappeared and its entry in my library was replaced by a DOS game from 1994 called Iron Cross (again, the game is on IGDB and until a few days ago appeared properly in my library).

So yeah, the feature is messed up and I wish they´d do something about it...