DuoBot does not speak Spanish (iOS)?
A couple of times I had a strange encounter when dueling with the duobot: "Couldn't start a match because your opponent isn't leaning Spanish." A little strange for a bot.
I always had to (manually) kill the application (not just closing) to resolve this issue:
edit: might be related to this: http://www.duolingo.com/comment/3798383 (yes, both reference each other).