Submitting HTML-form works sometimes and sometime not. Irregular behaviour

Hi!
The following problem has cost me a lot of time, so I would be very happy with your help…

I have an HTML-form with a submit button. I placed a simplified version at (http://www.groeiedag.nl/inschrijven-test/).

If I press the ‘Send’ button, sometime I am correctly redirected to the php-form which sends the e-mail, but some of the time nothing happens. Pressing multiple times will ultimately send the form.

I switched off plugins that could give this behaviour.
What am I doing wrong here?

The code of the form is below, I placed it straight in the Text-Editor.
(The hidden fields are necessary for the mail-php.)


<form action="/wp-content/themes/twentythirteen/formmailbasis.php" method="post" name="inschrijving">
<input maxlength="30" name="firstname" size="20" type="text" />
<input name="recipient" type="hidden" value="website@groeiedag.nl" />
<input name="subject" type="hidden" value="inschrijving" />
<input name="redirect" type="hidden" value="/dankjewelinschrijving/" />
<input name="email" type="hidden" /></td>
<input name="Button" type="submit" value="Verzenden" />
</form>