So this is a strange one. Whenever I search for "Earthbound" in the manually add games list I get an error. This only happens for earthbound. Restarting the program or my pc does nothing. The game appears fine on the IGDB database so I have no clue why this happens. Can anybody else replicate this? Is there a way to fix it?

Same here :/
Also, I can‘t search for Earthbound Beginnings either; it loads some time and then an error appears saying 'Sorry, we couldn’t load the data'

Edit: After some additional search I found this topic describing the same problem:
www.gog.com/forum/general_beta_gog_galaxy_2.0/adding_games_manually_search

Yes, I also facing the same consequences.

temporal fix

clear the search box 1st then start typing your game name

Having the same issue. Trying to add Earthbound Beginnings, so the workarounds don't work.