I'm trying to claim the King of Seas, but everytime I click the Add to library button the login screen pops up, but even after logging in it won't claim. Anybody else have this issue? Tried a few things, mostly disabling adblockers et al, even switching from Firefox to Edge, no dice... Did manage to claim the previous one using Edge i think, but even that fails r.n...

Try the direct link:
https://www.gog.com/giveaway/claim

I'm sorry, it's my fault. I claimed all the copies. All of them.

But rest assured, I plan on playing each of the 1.7M copies I got.

Simaltaniously ?...

I had the "log in" screen also pop up (despite being logged in already at that time), but after "logging in" a second time, I had no problems adding and downloading the freebie.

(Win 10, MS Edge)

Visiting the giveaway/claim uri gives an Already Claimed message, and even though the frontpage still shows the button it does appear in my library (with new tag)... oh whell...

Are you completely sure that you never owned it here before the giveaway ?...

Of course! It's the best way to check multiple endings.