Ubisoft has shipped some European copies of Rocksmith for PC without activation codes, leaving purchasers unable to play the game.
"Some physical copies of the PC version of Rocksmith are encountering problems during installation," the publisher admitted.
Disgruntled players in search of answers took to the Ubisoft forums and were redirected to a support page.
It requires users to fill out a form and provide a "clear digital image of your receipt clearly showing the store details and the product" as proof of purchase.
Ubisoft has apologised for the blunder and is sending out replacement activation keys. It's also offering affected customers free DLC: "We apologize for the inconvenience caused and will provide any affected consumers with the 'Guitarcade timesaver,' which gives players instant access to all the Rocksmith mini-games," it said.
Ubisoft has sued The Black Eyed Peas for $1 million for alleged breach of contract with regard to an unreleased iPad and iPhone game.