Well, I'm having the same problem. Anybody NOT having this? guessing its serverwide. It cant' be a change in multi mc, the vresion is the same as it's been for the last forever. It's not minecraft itself, loading via minecraft.exe works, as does play offline in MultiMC. Therefore, it has to be a problem with minecraft server authentication. Could be mojang blocked it, or they're just temporarily doing something with the servers. will post back if anything changes from here, but i suspect it'll be down till the creator can implement a fix, although *ninjaed* given the above post of a tweet, really thinking its going to be down until the MultiMC creator can implement a fix (hopefully fix anyway).
Same as Above, I had gone to sleep afk loading my Bc quarrys on my server, Came back to 0 FPS. When i wehn ion offline mode it worked Perfectly, but trying to login on my account it wouldnt again. Invalid response. My Normal Launcher works though
Edit 1 MMC still is not useable till it gets the above fixed
At least I know I'm not alone..phew ;-) Noticed I could still get on with minecraft.exe and thought I might have lost all my multimc worlds to a currupt file or something. (backups ftw there of course).
Oh well, off to some vanilla 1.3.1 until I can get back on my main.