This problem should only be temporary, so please try again after at least 20 minutes. It's possible that we were updating the game files while you were performing this action.
If this doesn't help, you should make sure that your security software (anti-virus, anti-spyware, firewall etc.) isn't interfering with GOG Galaxy. To do this, make sure that GOG Galaxy is added to the software's trusted applications list (how to do this varies, depending on the security software).
If needed, please try the following troubleshooting steps:
- Shut down GOG Galaxy and launch it again,
- Use MORE -> REPAIR on the affected game,
- Restart your computer (you'd be surprised;)),
- Uninstall Galaxy, and redownload the installer from here. After the download is complete, right-click on it and select Run as Administrator from the drop-down menu, and then install to the default location: C:\Program Files (x86)\GalaxyClient.