I would try to reinstall Form Builder then. You absolutely need the folder and path there mentioned for the extension to work at all.
I, too, experienced the same problems you mentioned when I installed Form Toolkit alongside my other extensions.
I can confirm that the fix Dani provided by replacing the shared.js clears that up. So, I would try the reinstall first and then if you have Form Toolkit installed, make the switch for the other file.
Best regards,
Brian