Try switching to the default theme and deactivating all other plugins.
I’m having the same problem. I’ve been using contact form 7 for years and never seen this. There is nothing unusual about the site I’m building, except that it is hosted on hostgator. Did you find a solution?
Solved it…
I have values set in the form fields, such as “Name”, “Email”, etc. So I use javascript to clear out the text inside the field when that field gains focus (the user tabs to or clicks in the field). Unfortunately, I did not make the selector specific enough so that it did not do this on the submit button, so the “value” of the submit button was being removed by javascript, which killed the form. I’ve made the javascript do this only on text and email fields, and that solved it.
