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I'm encountering an odd bug seemingly confined to how GOG Galaxy treats this game. Upon downloading and installing it, I'm greeted with a message that says "Installation Failed." If I do a Verify/Repair, the application will work for a second or two, and then default back to that same message. Every time I close the client and re-open it, it tries to re-download the game for about half a second, before once again settling on "Installation Failed." It also grays out the "Show Folder" option.

The thing is, though, the installation is fine. I'm able to play the game if I navigate over to the folder (all the files are right there, where they should be) and double-click on either the DX9 or DX10 .exe files.

This is a small matter - the only thing it prevents me from doing, as far as I can tell, is launching the game from the client's "Play" button, and keeping track of the hours I've spent with it. Still, though, odd and frustrating.
Same problem....
Yep, same problem here.
Assassin's Creed is my first game I bought on GOG. And I also have th esame problem.

Is there any support for GOG games? How to fix it?
I'm having this issue as well.
The Galaxy app just said me "Assassin's Creed Director's cut successfully installed".

Aye, same here.
Glad to see that some other people are getting this to work, but I'm still having the same problem.

At least now, though, it's a bit more specific on what the problem is. The error now says, "INSTALLATION FAILED: CONFIGURATION ERROR."

I wonder if there's any chance this is because I'm not keeping it on my "C" drive ... ?
Just want to clarify that above I metioned about having the same problem, not the solution. I hope the GOG team will eventually look into that - this is the only game so far that I can not launch on this platform.
I'm not sure about this solution:
1) I installed RU language pack;
2) I made a backup copy of the game.

After point 1 I also re-checked files of the game. The game was downloaded again. Still had the problem
I think that making a backup copy solves the problem somehow. I made a backup and re-checked (fixed) files. Try it. Hope it will help you too.

Or maybe I installed the game from a backup copy. Don't remember.
Hmm. Just opened the GOG Galaxy client for the first time in about two weeks and the problem seems to have fixed itself. I never tried any of Matumbable's recommendations ... it just finallly accepted the installation, on its own.