The Meaning of Life, the Universe, and Everything.
I have been looking into custom advancements and I got them to work on single player. I'd like to try to load these advancements on a server, preferably Spigot. "Installing" these advancements as I did on single player, doesnt seem to work for the server (pasting the advancements inside the world folder). This makes me question if its even possible on a server.
Perfectly possible, there's no difference at all between the two. Find the world directory (world save), then the data\advancements directory and put it all right in there. Just be careful that there's a difference between /reload and /minecraft:reload, you'll want the latter to reload any changes in advancements, functions and loot tables.
If you're using MultiVerse then the key is to use the default world save folder, so the one which got mentioned / setup in your server.properties (level-name).