Can't update a game in Galaxy that I own because I don't own a valid copy of it?

I bought the game on OCT 1.

What do I need to do to make my game a "valid copy"?

Long story short - They've accidentally added the upcoming, but unfinished Call of the Paragons DLC to our libraries.
Now, they removed the DLC which leads to that error message. The only way to update Asterigos is uninstall, then re-install.

Be aware that you can't load your saves anymore after the update because of the missing DLC. I've reported this issue to GOG and the devs, but it seems that nobody cares. :(

Filing a support ticket my self as I read this.

Edit: deleted most of this post since the mystery's solved.

If the problem persists, open a support ticket.
GOG GALAXY Installation failed (E6)/Update failed (E6) - You don't own a valid copy of this game - GOG Support

Thank you!
Maybe you've more luck.

So now I've uninstalled and reinstalled the game, but my save shows I am missing a DLC I never installed. I'm really getting tired of this store. Support ticket in for that.

Too bad it isn't like Morrowind, where it complains, and you say 'okay' a few dozen times for missing references, then save a new slot and reload it to get it clean...

Though games REALLY should look to see if you touched any other content before bothering to complain about it (or silently ignore it and remove the unneeded references). Especially new games.