I have the same problem as others.
I own the Collector's Edition of ED (German Retail, distributed by dtp), but I can't seem to find a Securom key. It's not on the manual or anywhere in the package. Also, there is no barcode on the back of the manual. I emailed support, but haven't received an answer yet.

Has anyone with a retail version successfully claimed a voucher? Thanks!

Yes, lots of people have gotten a voucher to upgrade to D2:DC.

Did you check the case under the DVD, etc? At least with the English version of D2:ED, all the retail and digital releases contain a Securom key, since Larian was unable to remove the copy protection (due to contractual issues). They did remove the DRM from DKS, so that is why some retail versions need a personalized picture of the back of the box/case (with the barcode clearly visible) as proof of ownership, if there is no Securom key.

Did you email support@larian.com? If so, you should have gotten an auto reply right away. They are at the start of the work day in Belgium now, so you should get a response soon (not sure what the current response time is, but it usually isn't more than a day or two, excluding weekends and holidays).

Thanks!
I have this version (attached images). The box (not the DVD box, but the large CE box) has a barcode printed on the bottom, but I doubt that's an individual one. It' an article number. So how would I "personalize" this?

The box was purchased at amazon.de.

Maybe get a picture of the collection as a whole, a close up of the DVD and back of the case, and one of the bottom of the box, etc, and include a hand written note in the shots with your name and email address you emailed support from, or something. Presumably with a readable bar code support can make sure they don't get different people sending in the same picture or different pictures of the same case, but I would think anything unique and detailed enough would be acceptable. You can always ask support for clarification.

Thank you, it worked!
Looking forward to replaying this title in an improved and extende version! :)