Hmm.
Your combined (& minified) JavaScript file is loaded in the header while some variables needed by the script are declared later, in the footer (where the script is supposed to load).
This way, the plugin script runs before the variable is declared, which is why it throws an error right now. It doesn’t know which form to scroll to.
Moving the script to the footer (which is recommended) should fix it. Or just do not include the script in your minify rules – it will only be loaded for people who submit your form anyway. You can configure this in the W3 Total Cache settings.
I will look for a way to workaround this as well.
PS. in W3 Total Cache > Combine & Minify, are you using Manual or Auto method?
Thanks!