Our team says that the launcher may be taking some time to update -- as long as your "additional content" page has updated and shows ownership of Astral Planes, you will have access to the new DLC.
Originally posted by Lucullo: I too have this problem. I verified the files and restarted Steam a couple of times but it is still there.
However, the "unable to verify ownership" flag appears only in the launcher (DLC tab) while the "addtional content" page says "owned".
I've not yet tried playing.
Our team says that the launcher may be a little wonky, but as long as the "additional content" page has updated, you will have access to the new DLC.