Gabriel

Hi, Just to give some light

The new WP release 3.9.2 has a important change related to the login area, cookies and hashes, this will broken any plugin that uses the old integration.

For what I see the error 500 is caused by a combination of plugins that try to use part of the login area in a loop or by internal http fopen file_get_contents etc without a good error handling. Try disabling any plugin or enhancement related to the login area, with the important plugins activated
ACF
Contact Form 7
Woocommerce
WP Bakery

The problem was the logic to include the serialized array into the radio buttons to select the images, with these changes the serialized data is well formated into the wp_options

I changed the logic from $image=str_replace(“\””,”\'”,serialize($image)); to $imagedata = htmlspecialchars(serialize($image));

And other changes related to the checked function

wp-content/plugins/memphis-wordpress-custom-login/libs/mwpl.dashboard.menu.php

function mwpl_get_bgimage_div($image) {
        $imageurl = $image['imageurl'];
        $imagepath = $image['imagepath'];
        $reg = unserialize(get_option('mwpl_custom_bgimage'));
        //$image = str_replace("\"","\'",serialize($image));
        $reg = $reg['imageurl'];
        $imagedata = htmlspecialchars(serialize($image));
        $nonce = wp_create_nonce ('my-nonce');
        ?>
                <div class="mwpl_bg_container">
                        <p class="mwpl_bg_container_header">
                                <label class="mwpl_bg_container_label">
                                <input name="mwpl_custom_bgimage" type="radio" id="<?php echo $imageurl; ?>" value="<?php echo $imagedata; ?>" <?php checked( $reg, $imageurl); ?> /> <?php _e( 'Make This Your Background.' ); ?>

in the wp-content/plugins/memphis-wordpress-custom-login/css/memphis-wp-login.php

I just commented the line

//$custom_bgimage = str_replace("\'","\"",$custom_bgimage);