I've had this problem too - although not with this site. In fact this site is one of the few I can upload to.
Having said that, I haven't uploaded anything here in the last week, so if the Admins have changed anything recently...

I can tell you, though, that with another computer connected to the same router, I can upload without trouble, so it's nothing to do with my connection. This could mean that either there is a fault with one of my computers, or (more likely IMO) that something 'out there' has changed in such a way that one computer can cope with the change but the other (older one) can't. The geeks are always adding something new that my old box of bits can't cope with.

Sorry, I can't help more than that - I just drive 'em, I dunno what makes 'em go.