I'm having the same problem. In fact, this problem is also visible on wordpress.com !
The problem is: you can't insert images as a gallery to a new post, through Quick Press or Posts -> Add New
You can only insert photos as single images, by pressing "Insert into Post".
The source of the problem is (as i understand it), the fact that a new post, while it is not yet autosaved as a draft, does not have an ID. So you can't attach new photos to it, because they need an ID of the post they are being attached to as their post_parent.
It is clearly visible, through Posts -> Add New. Try to add photos as a gallery right after you open up that page, you wont be able to do it. After pressing "Save all changes" the content of the Media popup will be blank, instead of a new Gallery tab appearing.
But, if you type in post title, then wait for the draft to be saved (you can know when it is saved when the line with the permalink appears right below the post title input), the media uploader will work as expected.
This is also why, media inserting as a gallery always works for edited posts (because they are already saved, therefore always have an id), and why it never works for Quick Press (because a post added through it never has an ID, because it only gets it when saved, and after you save it you can only edit it through Posts -> Edit).
Inserting images directly into the post (one by one, not as a gallery that is) works, because they are directly inserted into the post contents and are not saved separately, therefore they don't need a post_parent.
My questions are:
- Was this always the case? I mean was this problem introduced in 2.8 or any other version ?
- Is there any way to fix this?