When defering JS: Uncaught TypeError: wpcf7.initForm is not a function

Hi!
I am running CF 4.9.2. As fast response times gain more and more importance I tried to speed my page up by comining, minifying and defering all JS. Currently, I use the W3 Total Cache plugin for that. However, independend of the plugin I receive the following console error on recipient side:

Uncaught TypeError: wpcf7.initForm is not a function

While the error disappears when disabling JS plugins, this is not a solution. I use tons of JS in themes and plugins, none on which causes any problems with page optimization tools.

Thus, I kindly ask to modify the plugin in such a way that this error is avoided when using any JS optimization tools.

The page I need help with: [log in to see the link]