503 means Service Unavailable, 500 is a slightly more generic Server error.
They're both likely to be intermittent when the server is under very heavy load, like it is lately with the reddit migration stuff going on.
Wait 5-10 minutes and refresh to try again. Try not to spam refresh every 5 seconds. If everyone did that, it would make the issue worse and more long-lasting.
Over time I'm sure we'll see server improvements :D