This error can occur after installing preview (beta) version and going back to the public version.
The solution is to close the Galaxy Client and remove the contents of the Storage directory.
On Windows it is here:
You can access the directory using the Run... window (WinKey+R key combination) and entering:
Please also remove this file:
On Mac it is here:
You can open it using the "Go to Folder" command accessible with the Command+Shift+G key combination or selecting Go > Go to Folder from the Finder menu.
After removing the contents of the Storage directory you can start the Galaxy Client again. Importing games will be required, you can do this by clicking on the GOG icon in the top left corner and selecting "Scan and import folders".