VM - custom payment plugin and payment methods do not work

Question

I can't make my VM payment plugin to work at joomla v3.5.0 and virtuemart v3.0.12. I've install a custom payment plugin. Initially it worked, yet the same time other payment methods (when invoked) were wrong directed to my custom plugin payment gateway.

Therefore I've slightly changed it. And now when the plugin is installed and turned on in admin panel, none of payment methods work. As I process with a cart (enter data, choose any payment method, etc.) and click on 'process' btn. the following error transpires: Error 500. Site can't process your request.

Also, the plugin's payment methods settings are not saved in admin (besides defaults):

how to break thru, any hints? You may ask additionally questions to clarify the case.

Update

I've not done many edits of the plugin code. Mainly small edits were in manifest file latcard.xml to make parameters savable. But to no positive result.

Manifest file:

<?xml version="1.0" encoding="UTF-8" ?>
<extension version="1" type="plugin" group="vmpayment" method="upgrade">
<name>VM - Payment, Latcard</name>

<creationDate>November 28 2015</creationDate>
<author>SIA LatCard</author>
<authorEmail>[email protected]</authorEmail>
<authorUrl>http://www.latcard.eu</authorUrl>
<author>LatCard AS</author>
<creationDate>November 2015</creationDate>
<copyright>Copyright SIA LatCard</copyright>
<license>http://www.gnu.org/licenses/gpl-2.0.html GNU/GPL</license>
<version>1.0.0</version>
<description>
    <![CDATA[<a href="http://www.latcard.eu" target="_blank">LatCard</a> ]]>
</description>

<files>
    <filename plugin="latcard">latcard.php</filename>
    <filename>latcard.xml</filename>
    <filename>index.html</filename>
    <folder>latcard</folder>
    <filename>controller.php</filename>     
    <folder>views</folder>
</files>

<vmconfig>
        <fields name="params">
                <fieldset name="basic" addfieldpath="/plugins/vmpayment/latcard/latcard/fields">
                    <fields addpath="/plugins/vmpayment/latcard/elements"/>
                    <fields addpath="/plugins/vmpayment/latcard/latcard/elements"/>
                    <fields addpath="/administrator/components/com_virtuemart/elements"/>


                    <field name="merchant" type="text" size="10" default="" label="Merchant ID" description="Merchant ID"/>
                    <field name="site_id" type="text" size="10" default="" label="Site ID" description="Site ID"/>
                    <field name="password" type="text" size="10" default="" label="SOAP password" description="SOAP password"/>

                    <field name="secure_3d" type="radio" default="1" label="3D Secure" description="3D Secure">
                            <option value="0" >No</option>
                            <option value="1" >Yes</option>
                    </field>
                </fieldset>
        </fields>
</vmconfig> 

Answer 1

500 error may come from your plugin's PHP code.

I got the same problem that my plugin's configuration was not saved. After debugging in Virtuemart code, I found out that I needed this function in my plugin's class:

function plgVmDeclarePluginParamsPaymentVM3(&$data) {
    return $this->declarePluginParams('payment', $data);
}

Ensure that you already setup for parameters and pass them to Virtuemart in __contstruct function. For example:

public function __construct(&$subject, $config)
{
    parent::__construct($subject, $config);

    $this->_loggable = true;
    $this->tableFields = array_keys($this->getTableSQLFields());
    $this->_tablepkey = 'id';
    $this->_tableId = 'id';

    $varsToPush = [
        'my_param' => array('', 'char'),
    ];

    $this->setConfigParameterable($this->_configTableFieldName, $varsToPush);
}