TheDoomBringer

I made an account just to say this, as I have found a solution. You need to go into the Tekkit modpack files find the coremods file then remove old files from it. If you're on Windows then the files should have a timestamp next to them. Remove any files not from today and then it will work properly. I think this is a problem caused by not running the launcher with administrative privileges so it can't delete the old files.