OVA Announced

According to Sneaker Bunko, the release of Dantalian no Shoka OVA was canceled. It was supposed to be bundled with the ninth volume of the original novel, but the release of the novel itself was also canceled. Kadokawa didn't reveal the reason of the cancellation.

Source: Kadokawa Sneaker Bunko

Updated
The novel author Mikumo Gakuto explained the reason of the cancellation is that there was a disagreement about the continuation of the novel series. The author and the editors had once reached an agreement to conclude the series by the eighth volume, but the editors suddenly asked the author to elongate the series to this Spring with nine volumes. The author turned down the proposal and the ninth volume was officially canceled. Mikumo said there has been no antagonism between Sneaker Bunko and him and that the OVA had nothing to do with the cancellation of the novel.

Source: Mikumo's blog