Our Galaxy Team is investigating the issue. The following workarounds should help.
1) Aborting the installation and import already existing files.
To do this you will have to abort the installation (clicking MORE > Manage installation > Verify / Repair and aborting it might be required) and use the MORE > Manage installation > Import folder option to get the Galaxy Client to recognize the files on your drive.
2) Installing to a different directory.
Simply selecting to install one more time, but to a non-default directory might work.
3) Using the standalone installers to install the game.
To do this in the Galaxy Client, click MORE > Backup & goodies and select the Game Backup Copy. After the download completes right-click the setup file and select to "Run as administrator" to start the installation process.