Wordpress backend form returns JSON - wordpress

Question

I developed a small one-file plugin for a wordpress site. It's just adds an upload form for a file that POSTs to itself and then does some stuff with the file's contents once submitted.
It works fine on the staging environment, but on the live server there is a strange problem. Once I submit the form with the file the server does not bring me back to the same page but instead returns the following JSON:
{"success":false,"message":"Please enter a message."}
I am at loss what could be causing this. Has anybody come across such a problem?
The (simplified plugin):
function show_upload_form() {
if ($_FILES['userfile']) {
echo "<p>file received</p>";
$file = fopen($_FILES['userfile']['tmp_name'], "r");
$data = [];
while (!feof($file)) {
$data[] = fgetcsv($file,null,';');
}
foreach ($data as $line) {
$pid = $line[0];
if (isset($line[1])) {
$price = trim(str_replace(',', '.', $line[1]));
} else {
$price = "";
}
if (isset($line[2])) {
$title = trim(iconv("ISO-8859-1", "UTF-8", $line[2]));
} else {
$title = "";
}
global $wpdb;
$product_ids = $wpdb->get_results($wpdb->prepare( "SELECT post_id FROM $wpdb->postmeta WHERE meta_key='_sku' AND meta_value='%s'", $pid));
foreach ($product_ids as $product_id) {
$elem = $product_id->post_id;
if ($price != "") {
update_post_meta($elem, '_price', $price);
update_post_meta($elem, '_sale_price', $price);
update_post_meta($elem, '_regular_price', $price);
}
if ($title != "") {
wp_update_post(array(
'ID' => $elem,
'post_title' => $title,
));
}
}
echo "<p>Produkt #".$pid;
if ($price != "") {
echo " - new price: ".$price."€";
}
if ($title != "") {
echo "- new title: ".$title;
}
}
} else {
echo "<form method=\"post\" enctype=\"multipart/form-data\">";
echo "<label for=\"file\">Select a file:</label>";
echo "<input type=\"file\" name=\"userfile\" id=\"file\">";
echo "<br /><br />";
echo "<button>Upload File</button>";
echo "<form>";
}
}
I appreciate any input.

Answer 1

Try something like this
<form action="<?=admin_url( 'admin-post.php' ) ?>" method="POST">
<input type="hidden" name="action" value="my_custom_plugin_action"/>
<input type="submit" value="SEND"/>
</form>
add_action( 'admin_post_nopriv_my_custom_plugin_action',array( "class_that_owns_that_function", 'show_upload_form' ) );
public function show_upload_form()
{
//Here write your code
}

Answer 2

Problem disappeared after deactivating and individually reactivating all the plugins for the second or third time. Guess this one will remain a mystery ...