Internal client error: java.io.IOException: Server returned HTTP response code: 503 for URL: http://session.minec...erver.jsp?user=
Is not a YAMS problem, but a Mojang/Minecraft one again, as you can see from here the session status is red: http://xpaw.ru/mcstatus/
Mojang have posted about the issue on their blog here: http://www.mojang.co...have-a-problem/
We are aware of the security issues (people using others’ usernames NOT any leak of passwords or personal information) involved with the Minecraft authorization servers and are currently working to fix it.
Right now the authorization servers have been taken offline and will be down until further notice. The Mojangstas are working hard to make sure we get everything back to perfect working order.
We’ll keep you updated as we have a more clear timeline. We really are sorry about this and are working as fast as we can!
I suggest you go and do something else and try again latter, as every other server will be having the same problem.
I will post as soon as I know it's working again!