When you log in to any of our mobile apps, the app transmits your user name and password to our servers using SSL (to protect your password from prying eyes).
In order for this to work your device must trust our SSL certificate. To be more precise, it must trust the cert of the Certificate Authority (CA) that issued our SSL cert.
By far the most common reason certificate validation fails is because the clock on your device is way off (e.g. the year is wrong). Certificates have a validity period, so if your clock is set forward or back many years (e.g. if it got reset somehow in a crash and is now sitting at 1970, for example) you’re going to have a heck of a time getting SSL to work anywhere, because no certs will be considered valid.
So the first thing to do is check the time on your device and make sure it’s current.
If it’s not your clock, the other possible problem has to do with the way certificates work in a chain. For SSL to work properly, your device must have a set of certificates for the major CAs on the Internet (Verisign, Thawte, GoDaddy, etc.) If your device were missing the root certificate for our CA, you’d also have trouble getting SSL to work with our website. All of the mobile devices we know of have our root CA installed by default.