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.
3. Open minecraft through the Technic Launcher and punch in your credentials. (username/password)
4. Relaunch your modpack or vanilla; whichever you play.
5. Connect to server.
NOTE: To prevent your computer from running out of memory if you have a 64 bit operating system. Uninstall your Java Script from your computer using the uninstall option form the website. Install 64 bit Java Script from their website under https://www.java.com/en/download/faq/java_win64bit.xml
This will explain to you which one you need and how to install it.
Open technic launcher and go to the right side of the screen there will be a button next to Launcher Options. Open this menu and go to the JAVA SETTINGS tab at the top. Under memory increase your memory to the maximum amount. Mine is currently at 7 GB.
Be aware increasing the amount of memory may cause your game to crash instead of close properly when your done playing for prolonged periods of time. To avoid this close your game every so often (1 1/2 to 2) hours to prevent this. I usually forget and I sometimes have to force close it but you won't cause any issues in the long run.