What version of the game do you have, retail? And did you mean to post this in the German forum?
I think all the download distributors have long since switched to the Developer's Cut release.
Depending on the version and when it was produced, there may not be an activation code for the game, or it may just do a disk check on install (in that case you could try a different optical drive, if available).

The easiest solution would probably be to get the Dragon Knight Saga DRM free patch.

Alternately, when the Anthology was released there was an offer to upgrade people who had the PC version of D2:ED or DKS to a download copy from the Larian Vault. Eventually that offer will expire, but AFAIK it is still good; see the topic Divinity 2: Developer's Cut and Anthology on Steam for details.