HTML form $_POST problem

I use a shortcode with the following PHP code on a page:

 function email_bereitstellen() {
	$id = $_POST["id"];
	$conn = mysqli_connect('rdbms.strato.de', 'U2884970', 'RISSim2017', 'DB2884970');
    // Check connection
	if ($conn->connect_error) {
		die("Verbindungsfehler: " . $conn->connect_error);
	};
	$query = "SELECT name, vorname, email FROM wp_gremien WHERE Id=$id";
	$result = mysqli_query ($conn, $query);
	if ($result!= TRUE) {
    	echo "Error: " . $query . "<br>" . $conn->error . '<br>';
	} else {
		$row = mysqli_fetch_assoc($result);
		$name = iconv('ISO-8859-1', 'UTF-8', $row["name"]);
		$vorname = iconv('ISO-8859-1', 'UTF-8', $row["vorname"]);
	    $email = $row['email'];
		$name = $vorname . ' ' . $name;
		echo '<h2>Gremienmitglied: ' . $name . ' ('. $email . ')</h2>';
?>
	<form action="http://huppenbroich.de/ris/gremien/pflege/"><button style="padding: 10px;" type="submit">falsche ID</button></form>
<?php
		echo '<br><h3>E-Mail Adresse für Bürger/innen freischalten/löschen?</h3>';
?>
	<form action="http://huppenbroich.de/ris/email-schalten/" method="post">
		<input type="radio" name="schalten" value="freischalten" checked> freischalten<br>
		<input type="radio" name="schalten" value="loeschen"> löschen<br>		
		<input type="hidden" value="
<?php	echo $name; ?>" name="name">
		<input type="hidden" value="
<?php	echo $email; ?>" name="email">
		<input type="hidden" value="
<?php	echo $id; ?>" name="id">
		<br><p><input type="submit" value="Verarbeiten"></p>
	</form>
<?php	
	mysqli_close($conn);
   }
}
add_shortcode('email-bereitmachen', 'email_bereitstellen');

When the action is invoked I get a 404 error. In the URL line the correct page is shown and when I click on edit page the page to be invoked is shown.

When I delete a hidden input line everything is okay. Then the correct page is invoked.

What may be the reason of the problem?

Another example:

function email_senden() {
?>
<h2 style="margin-bottom: 20px !important;">Ihre E-Mail an Empfänger</h2>
<form action="http://huppenbroich.de/ris/email-absenden/" method="post">Mein Name <span style="color: red;">*</span>
<input name="name" required size="40" type="text" />
Meine E-Mail <span style="color: red;">*</span>
<input name="email" required size="40" type="email" />
Betreff
<input name="betreff" size="40" type="text" />
Nachricht <span style="color: red;">*</span>
<textarea cols="30" name="nachricht" required rows="10"></textarea>
<input name="empfaenger" type="hidden" value="hans@keutgen.com" />
<input type="submit" value="Absenden"></form>
<?php
}
add_shortcode('email-senden','email_senden');

This is only the starting point of a larger code sequence. The effect is just the same: 404 and correct URL.