I recently have had a error using the Minecraft.net/play. I can play on single player, but when it comes to online playing, it just gives an error when trying to join. (Failed to login: bad login). I do have the actual client, and it works perfectly on it, but I can not download anything on the computer I am using. I looked up how to fix this, and only found things 2 years or older. I did everything they said to do, (Log out on all other computers, reset password, ect.) It did nothing. But just recently I tried another account owned by someone else, and it worked. Difference? He had a old account that logged in through a user name and not an email. So i'm assuming the Mojang accounts, (Log in with email) are to blame for this error. I would appreciate it of someone was kind enough to tell me a solution to this problem.
-Randivous_
Some other people have reported an issue with multiplayer on the website. Your best bet is to just use the downloaded client.
Stu