According (http://wordpress.org/support/topic/plugin-contact-form-7-this-is-how-to-showhide-fields-with-jquery?replies=39) and my additional solution to use more condtional hiding fields I came up to another problem.
When is fullfilled to a certain condition there will show up fields you have to fill in. The data to fill in must be filled in. So I make these [text] and [select] fields required to fill in. Add * ([text*] resp. [select*]).
However when this condition is not for use these fields does not show up. Because they are required to fill in it is not possible the form to sent.
It is very easy don’t use required fields (so I did), but if then a showed up field is not filled we don’t have the right data sent in.
Have somebody an idea how to solve this problem?