Here's my crash report, with the same problem. only this happens consecutively. Currently it has crashed 4 times today, yet was working fine this morning.
Yesterday i've had no issues, and the day before last it repeatedly crashed. it launches just fine, and loads into singleplayer but it won't go into multiplayer,
this is on multiple servers not just the one. i'll click login and it goes like normal, in the console i can see people chatting, so i'm in the game and then it crashes.
when i switch modpacks i have no issues logging in whatsoever. please help?
edit: here's another, just for kicks.