Facing issues after using Magento 2.3.3 p1

Using Magento 2.3.3 p1. Applied this security patch to Magento 2.3.3.

Now we are facing issue to save Magento configuration for specific module with options. Throwing 503 error. It was working fine for Magento 2.3.3. Is this issue with Magento 2.3.3 p1 ?

system.xml

<group>
...
...
...
<field id="selectourexp" translate="label" type="select" sortOrder="2" showInDefault="0" showInWebsite="1" showInStore="0">
                    <label>Our Integration</label>
                    <source_model>NamespaceModuleModelSystemConfigOurexp</source_model>
                    <backend_model>NamespaceModuleModelSystemConfigOurbackendcall</backend_model>
                    <depends>
                        <field id="ourflag">enabled</field>
                    </depends>
                </field>
            </group>

Ourexp.php

<?php
namespace NamespaceModuleModelSystemConfig;

class Ourexp implements MagentoFrameworkOptionArrayInterface
{
    /**
     * @return array
     */
    public function toOptionArray()
    {
        return [
            [
                'value' => NamespaceModuleModelCallLayer::FRONTEND_JAVASCRIPT,
                'label' => __('Option 1')
            ],
            [
                'value' => NamespaceModuleModelCallLayer::BACKEND_API,
                'label' => __('Option 2')
            ],
        ];
    }
}

Not defining anything related to options in Ourbackendcall.php