I had to install a Rocksmith 2014 update yesterday, it wasn't a Steam update, it was an update for the game. And exactly after this update Rocksmith stopped working, I can get it running with that cDLC dll file. I wanted to try to roll back the game to an older version, but it's not possible, I probably need a beta tester code to get that roll back function on steam.
So Ubisoft 100% screwed us again and forcing us to go to Rocksmith+ with their dumb subscription fees...