After getting this message and reading through these posts, I uninstalled GOG Galaxy then ran the registry scanner within CrapCleaner.
Had it back up the registry, "fix selected issues" (which seem to mostly delete obsolete keys), and reran the GOG Galaxy installer.
GOG Galaxy is now running, scanning all my hundreds of games back into itself.
So far, so good. And I am relieved I didn't have to hunt through the registry looking for GOG and Galaxy...

Update: after several hours Galaxy was still scanning. I went to work, and came home for lunch. Galaxy was flashing between a gray and a white screen. Once I got it shut down through the task Manager, I tried to restart it. Back to the same error message...

I have company coming this week; maybe in a couple of weeks I can spend hours messing with this, but for now, Galaxy is going to be uninstalled.

Just wanted to throw in that I am now having this issue as well. It was working fine last week.
I uninstalled, cleaned and attempted to reinstall. Now the installer locks up and will not finish.
Don't know what else to try.


UPDATE 09-24-2016

OK I finally got Galaxy working again.
First I had to completely uninstall Galaxy.
Next I searched for everything Galaxy and GoG related on my C: Drive and external drive where my games are installed.
I deleted everything except for my games folder.

The next step is very risky so do NOT do it if you are not comfortable. Seriously Don't even try unless you have done it before or can find someone trust worthy who has done it before.

I searched and edited my registry to remove everything Galaxy and GoG related EXCEPT the parts pointing to my games directory. I would suggest a program like Registry Workshop to make this process much faster.

Finally I used a registry cleaner program (System Mechanic Pro) to clean and compact the registry.
REBOOT
Reinstall with a freshly downloaded Galaxy installer.
Hey my library is finally back.
Now if I could figure out how to enable the Gwent Alpha.

I'm also having this problem.

Today I wanted to install the Gwent Beta, it updated GoG Galaxy. When I try to start GoG Galaxy now, it says: "Failed to load game database"
Any news on how to fix it?

Have the exact same problem, already contacted support. They suggested to delete galaxy.db file from C:\ProgramData\GOG.com\Galaxy\Storage\, but sadly it didn't work for me. Waiting for their next reposne, will post any tips I get.

Its a problem with permissions in win 10 and 8. Easy way to fix this problem is install gog galaxy not in standart C:/ folder. After I installed it into D drive it started to work fine.

Well, I'll be damned... That did it! Kudos to you, neighbour!

I reinstalled it (in C:/...) and now it works again, but thx :D

so i deleted C:\ProgramData\GOG.com\Galaxy\storage\galaxy.db and index.db and gog galaxy did boot up. Its downloading gwent now as I started the installation from my Games Library in the GOG-Galaxy-Client.

IT WORKS =)

nm

I know thats a probably stupid question but how did you install in on the D drive?

Same problem!!! I can't open the game... And i tryed install at the c and d... =/

Possible Solution:

Uninstall GOG Galaxy > Disable AV > Re-download GOG Galaxy > Change default install directory.

I downloaded the GoG Galaxy through the download window you got when redemming GWENT and that failed so i went to the main site and downloaded first the Launcher and that worked for me

Does anyone have a fix for this on Mac OS X 10.9?