Try making a clean install of GOG Galaxy.
First uninstall GOG Galaxy by running the unins000.exe file located in the GOG Galaxy installation folder (by default C:\Program Files (x86)\GalaxyClient ).
Then make sure that following folders are also removed:
* C:\Users\All Users\GOG.com,
where *username* should be your Windows Account Username.
You made need to show hidden files and folders in the "Folder and search options", if you haven't done so already.
Restart your computer. Then download the latest version of GOG Galaxy from our website:
and run the installation as administrator.