Layout problem on signup page following update


Following one of your plugin updates (I think New Blog Templates, but I can't be sure), I now have a problem with the layout on my signup page.
The domain drop-down box has somehow dropped to the bottom of the page, instead of being aligned with the "Site Domain" field:

This happened once before after a New Blog Templates update, but was resolved in a later update.

The problem does not occur with the twentyfourteen theme.

Any ideas?