I too am having this problem. It just started today too. I've wiped everything and restarted my computer. Vanilla mincraft seems to work fine but not tekkit. Can anyone tell me whats going on and what i need to do?
I've also been having the same problem, starting today. I was on a server, all fine and dandy, as always. Then, the server reboots, as it does systematically, and now I can't go on any tekkit servers, or any feed the beast servers without getting the bad login error. However, regular minecraft works flawlessly.
Are you guys using the new technic launcher?
If you are, all you should have to do is open it and click on log out.
Then DO NOT SELECT remember me and log in again.
Worked for me, i hope it'll help you guys too.
It's caused when you open up two launchers and you login on both. It thinks someone is hacking so it tries to make minecraft not work ( turns into cracked) same happened to me and my friend my friend cant connect but I can sometimes, I know its a sad problem hopefully mojang will find a solution to this problem. So far there are no solutions.