We have a service in place to help keep the site online when this happens and it checks your IP address to make sure you are logging in from North America and not some overseas address (where most ddos attacks originate). The browser checking service has some problems with mobile devices and so you may be having trouble logging in. We are sorry for the inconvenience but it's unavoidable and with a little time this will blow over.
Curiously, I am typing this message from the same device, just using Safari, instead of the app. That is why I suspected that the problem was unique to the app.