I played the game yesterday now it fails to start because of serial key. First time I have DRM problems in a gog game!

Are all games in gog truelly DRM free or not?

You haven't said which Call of Juarez game this is (there are three on GOG).

GOG games are either DRM free or have inert (non-functioning) DRM. In the latter case, this can be an issue if you have ever had a previous DRM'd version of the game installed as the inert DRM can pick up registry entries and such from the old installation which can then make it live.

Regardless, you should contact GOG Support to investigate. Two of the CoJ games are new arrivals at GOG so there could be a mistake made with the packaging that they need to investigate.

Sorry that was a fail of me, the game was the first Call of Juarez and yes I add the boxed retail version installed ages ago. I reinstalleed the game again and it works now, I even used CCleaner to clean the registry, lets hope this solves what caused the problem.

Like I said, I never add this happen before, so it was a surprise to me, I always thought the DRM was always removed entirelly and not just made "inert" that sounds like asking for trouble.

Hmmm, I just had the same thing. I have never before installed any version of Call of Juarez, so this is definitely not the issue here. Played just fine yesterday, and today I get this message.

Just deinstalling and reinstalling the game did not fix the issue for me... so I will attempt to clean the registry. But this is not what a well behaved game should do...

GoG help support team advised me to try to play the game with administrator privileges and that solved the problem for me, hope it does for you.

Yes, they told me the same thing, and that works for me.

So, in order to fix this once and for all, I opened the game directory, located the "Run Call of Juarez" shortcut. Right clicked on it and opened the settings dialog. Switched to the Compatibility tab, and there switched on "Run as Administrator". Now I can also launch the game through GOG Galaxy again.

Strange, though, that this is needed for some people. Does this mean that everybody else runs their games as administrator all the time? (rethoric question, obvsly)