I would check their forums, this seems to be a big thread on the issue: https://www.gog.com/forum/general_beta_gog_galaxy_2.0/not_possible_to_download_a_game_via_gog_galaxy_problem_server/page9 Maybe disable IPv6?

GOG.com also allows you to download game installers directly, if nothing else that should get you going.

What kind of server issue? But generally the only way to resolve this is to try again later or contact the site's support.

There's not a lot that can be done if it is a real server side problem (500 range of HTTP errors.).