So Ubisoft support finally got back to me, with a friendly excuse for beeing so late due to heavy workload, but couldn't offer any real help other than "informing the apropriate developers" and the promise to get back to me after "recieving an answer from them". We'll see.
I also got a copy/pasted reply from them. They told me the developers were notified and are investigating the problem but they don't know when it'll be fixed since that information is not shared with them. Which to me is just a polite way of saying they don't know what the problem is and can't be bothered to actually address it. I know I didn't spend a bunch of money on the game, it was only a few bucks, but for me it's about not having legit access to content I paid for no matter if it's 1 dollar or 100 dollars.
Some news, Ubisoft support got back to me and asked me to upload screenshots proofing my Humble Store purchase and of the key for the game to forward to the developers. I did that and now lets see what happens next. I am a bit worried, as there seems to be a general problem with the ingame store, but if they'll manage to unlock the DLC for me (or us of course) permanently I'm simply fine with that. ;-)
Hey guys! First, please make sure your Steam account is linked to your Uplay account. If it is, please try verifying your game files through Steam. If you still cannot access the DLC content, please create a ticket with screenshots of proof of purchase for your game and the DLC content. Our team is currently investigating this issue.
Thank you to those who already have tickets in! I will be forwarding any tickets that are delayed for a response to the correct department once you've added the appropriate information. Thanks again!
Checked if STEAM account is linked, verified files and ticket is in since January 14th.
Tiny little update though, I asked for a status update for my ticket and got a reply from another customer support person telling me that "he took over my case" and that "he now forwarded" my uploaded files to the developers (I uploaded the screens 12 days ago) and will get back to me once they get back to him. Also the status of my case is now "forwarded" instead of "working on it". So more wait time. Lets hope this gets solved soon.