Thank you for the debugging
I changed something in that add on. The main change is here (changing from action to filter)
Apparently not all theme supports this new filter :
'comment_form_default_fields'
I am going to change that and go back to the old way (may be just restore the v6.0 to 7.0 as you suggested)
Have a nice day
< Version: 6.0
---
> Version: 7.0
20c20
< add_action( 'comment_form', array( __CLASS__, 'comment_form' ) );
---
> add_filter( 'comment_form_default_fields', array( __CLASS__, 'comment_form_default_fields' ), 8, 1 );