i have purchased little nightmares I and II on a sale yesterday and as i usually do, i will download the backup installers.
I can download them for the second game, but the first one returns an xml error saying i'm missing a key.

i was suggested to refresh my account but all this does is load my library and i still can't download the installer.

How can i get my hands on the installer then?

It's a problem on the gog servers. You have to wait until it will be fixed next week (hopefully).

You should keep the old installers right now, especially if the presumption here is correct.

i don't have the old installers as i purchased the game yesterday, and yesterday i couldn't download any file but the first one.

Downloading from the browser works fine.

No, it doesn't.

btw. this gets already discussed in
https://www.gog.com/forum/general/release_little_nightmares_i_ii_bundle_with_a_discount_and_2_dlcs_for_the_first_game_77785

Really? So this is game related issue. Because games I had the same issue as OP on Galaxy work on the browser.

Thank you for letting me know.