In a new post, GRRM shares some information regarding the state of A Dance with Dragons. The post does contain vague spoilers for both ADwD and the following book, The Winds of Winter, so proceed with caution. Here is the spoilery post.
In brief: in one of his many restructurings of the timeline of events in the novel, he has moved an important event from near the end of the book to the late middle ... and now has moved it back. This has meant that he’s been able to move chapters for a previously-unstated POV character (which he names) to the next book, as well as a partially written chapter and an as-yet-unwritten complimentary POV chapter. It’s quite clear that the timeline of this novel has been a major, major obstacle.
On the downside, for those who want A Dance with Dragons to be extremely long, this shifting of chapters to the next book reduces the length somewhat. On the plus side, he now has some possible chapters done for the next book, and may be that much closer on finishing ADwD ... provided that he doesn’t change his mind again.
It’s no surprise GRRM titled his post, “Dancing in Circles?” Good luck to him sorting out the rest of the niggling details!