Does this still happen ? I can't install my DLC . I just purchased Hearts of Stone and can't install it.
Is there another place to download it. It seems to be a constant problem with GOG.

I have the same problem wtf, did you manage to fix it?

Did you want to install HoS through Galaxy? If so, download the installer from the gog.com website and use that. Otherwise try Galaxy. Alternatively you can also use the gog downloader.

Last but not least you can file a support ticket. If it's indeed a server issue there's nothing you can do but wait.

Yeah I've been trying to patch The Witcher 3 and Install HoS through Galaxy. It keeps failing.

Can't get past patch 1.10 its really annoying. should have got the game through Steam. lol
Can't install HoS

I filed a support Ticket no response yet its been 2 days so I guess its going to take awhile before they fix it.

Have you tried downloading and installing manually. I ended up uninstalling Galaxy, downloading the files for W3 and the patches/DLC manually and installing. Worked for me. Galaxy is still Beta so has some bugs.