Problem Fix:
I will try to explain this as best i can, as i am not an expert.
This is what worked for me so i haven't tested it else where.
I have learned that this is because of the new Minecraft Launcher. For some reason it gets the jars mixed up. So to fix try moving your .minecraft folder to another location (maybe your desktop) and then start minecraft.exe (this will create a new .minecraft folder in roaming) then log in and have minecraft install. For me, i had to delete the tekkit folder and start from scratch but it might be different for you. From there running tekkit from scratch worked.
hope this helped.
