Thanks for letting us know. We're working on this now and will have a fix as soon as possible.
It turns out that the problem was a data issue. Specifically, there are a few cards that cause trouble when deleted. The most obvious of these is "*all+*default" which represents the default template for all new cards. Apparently that card got deleted on July 5th.
Obviously, it should not be possible to delete cards like this. We had written code to make it impossible to delete that card, so clearly something has gone wrong. I will look into the problem right away and make sure we have robust tests covering the issue.
In the meantime, I restored that card from the trash, and the site appears to be functioning fine again.
