It is hard to give specific help, since you're not providing any details about the client code... I'm able to create issues via mc_issue_add() on my dev box without any problems.

SOAP API normally throws SoapFault exception when something goes wrong; the error could be client- or server-side. Are you catching the exception ? Then you can call getMessage() to learn more about why it failed.

Check your webserver's log for errors. You can also try setting $g_log_level = LOG_WEBSERVICE; and see if there's anything in MantisBT's log file on the server.

There are no errors in logs.
I am using NuSOAP library for client

The full part of code is this:

   $ISSUE_PROJECT = (int)$_POST['SelectProject'];
   $ISSUE_CATEGORY = $_POST['SelectCategory'];
   $ISSUE_TITLE = $_POST['Title'];
   $ISSUE_DESCR = $_POST['Description'];
   $ISSUE_EMAIL = $_POST['email'];

   $ISSUE_ARRAY = array (
        'project' => array (
          'id' => $ISSUE_PROJECT
        ),
        'category' => $ISSUE_CATEGORY,
        'reporter' => array (
          'id' => '2',
        ),
        'summary' => $ISSUE_TITLE,
        'description' => $ISSUE_DESCR,
      );

    $mc_issue_add = array (
        'username' => $USERNAME,
        'password' => $PASSWORD,
        'issue' => $ISSUE_ARRAY,
    );
    $client = new nusoap_client($WSDL_POINT, false);
    $new_issue = $client->call('mc_issue_add', $mc_issue_add, $WSDL_POINT);

    if ($client->fault) {
        echo '<p><b>FAULT: ';
        print_r($new_issue);
        echo '</b></p>';
    } else {
        $err = $client->getError();
        if ($err) {
            echo '<p><b>Error: ' . $err . '</b></p>';
        } else {
            print_r($new_issue);
        }
    }

EDIT (dregad) fix markdown

Is there any particular reason for using NuSOAP for the client ? We gave up on that long ago because the library was unmaintained and not supporting newer PHP versions, and switched to using native PHP SOAP extension.

Can you try with this snippet, see if that makes any difference in your environment ?

<?php
$u = ...;
$p = ...;
$h = 'http://localhost/mantis/api/soap/mantisconnect.php?wsdl';

$mantis = new soapclient($h);
$t_issue = array(
    'project' => array( 'id' => 1),
    'summary' => "Test Mantis-SOAP-API",
    'reporter' => array( 'id' => 1 ),
    'description' => "Test",
    'category' => 'General',
);

try {
    $result = $mantis->mc_issue_add($u, $p, $t_issue);
    var_dump($result);
} catch(SoapFault $e) {
    var_dump($e);
    exit(1);
}

Is there any particular reason for using NuSOAP for the client ?

Actually no ) I'm not a programmer so i tried to do what i need based on some manuals.
Some years ago the development of frontend called anonymantis was abandoned, so i decided to create my own (if you interest in it - https://github.com/intervisionlord/mantis-reporter )

I'll try your suggestion, please give me some time, thanks.

Yes, your solution works fine, thanks a lot

Thanks for the feedback, glad I could help