403 Error when text is pasted in Custom Metabox Textarea

I gave your code a look over, and updated it to include proper escaping and sanitation. I suspect that the problem could have stemmed from the lack of escaping. I also applied some formatting and WP best practices, but otherwise the code is the same.

/**
 * Meta box display callback.
 *
 * @param WP_Post $post Current post object.
 */
function wpse_metabox_callback( $post ) {
    $service_title = get_post_meta( $post->ID, 'wpp_service_title', true );
    $service       = get_post_meta( $post->ID, 'service', false ); // get as an array ?>

    <div id="wpp_meta_inner">
    <div>
            <div class="wpp-input-container">
                <label><?php esc_html_e( 'Title', 'textdomain' ); ?></label>
                <input type="text" name="wpp_service_title" id="wpp_service_title" value="<?php echo esc_attr( $service_title ); ?>" />
                <p><?php esc_html_e( 'Will display default "Scope of Services" if empty.', 'textdomain' ); ?></p>
            </div>

            <div class="wpp-input-container">
                <label class="wpp-label-repeater"><?php _e( 'Service Items', 'textdomain' ); ?></label>
                <?php
                    wp_nonce_field( 'c_nonce_field', 'c_wpnonce' );
                    $c = 0;
                    if ( count( $service ) > 0 ) {
                        if ( ! empty( $service ) ) {
                            foreach( $service as $service_item_val ) {
                                foreach( $service_item_val as $service_item ) {
                                    if ( isset( $service_item['title'] ) || isset( $service_item['service_item'] ) ) {
                                        printf(
                                            '<div class="wpp-repeater-wrapper service">' .
                                                'Title: <input class="wpp-repeater-input service" type="text" name="service[%1$s]403 Error when text is pasted in Custom Metabox Textarea" value="%2$s" />' .
                                                'Description: <textarea class="wpp-repeater-input service" name="service[%1$s][service_item]" data-gramm_editor="false" value="">%3$s</textarea><span class="wpp-item-remove service">%4$s</span>' . 
                                            '</div>',
                                            esc_attr( $c ),
                                            esc_html( $service_item['title'] ),
                                            esc_textarea( $service_item['service_item'] ),
                                            esc_html( __( 'Remove', 'textdomain' ) )
                                        );
                                        $c += 1;
                                    }
                                }
                            }
                        }
                    }
                ?>
                <span id="services_here"></span>
                <div class="wpp-item-add services" style="visibility: hidden; margin-bottom: -20px;"><?php esc_html_e( 'Add Item', 'textdomain' ); ?></div>
                <div class="wpp-item-add services add-button"><?php esc_html_e( 'Add Service', 'textdomain' ); ?></div>
            </div>
        </div>

        <script>
            var $ =jQuery.noConflict();
            $(document).ready(function() {
                    var count = <?php echo esc_js( $c ); ?>;
                    $(".wpp-item-add.services").click(function() {
                            count = count + 1;
                            $('#services_here').append('<div class="wpp-repeater-wrapper service">Title: <input class="wpp-repeater-input service" type="text" name="service['+count+']403 Error when text is pasted in Custom Metabox Textarea" value="" placeholder="" />Description: <textarea class="wpp-repeater-input service" name="service['+count+'][service_item]" data-gramm_editor="false" value="" placeholder=""></textarea><span class="wpp-item-remove service">Remove</span></div>' );
                            return false;
                    });
                    $(".wpp-item-remove.service").live('click', function() {
                            $(this).parent().remove();
                    });
            });
        </script>
    </div><?php
}

When saving the metabox data, wp_kses_post() will be applied to each element of the $services array:

/**
 * Sanitize and save metabox data.
 */
add_action( 'save_post', 'wpse_save_meta_box' );
function wpse_save_meta_box( $post_id ) {
    if ( defined( 'DOING_AJAX' ) AND DOING_AJAX ) {
        return;
    }

    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

    if ( ! isset( $_POST['c_wpnonce'] ) || ! wp_verify_nonce( $_POST['c_wpnonce'], 'c_nonce_field' ) ) {
        return;
    }

    $allowed_html = array (
        'a' => array(
            'href' => array()
    ) );

    $service = isset( $_POST['service'] ) && ! empty( $_POST['service'] ) ? array_map( 'wp_kses_post', $_POST['service'] ) : array();

    update_post_meta( $post_id, 'wpp_service_title', wp_kses( $_POST['wpp_service_title'], $allowed_html ) );
    update_post_meta( $post_id, 'service', $service );
}

For the sake of completeness, here is the code that registers the proposal post type:

/**
 * Register proposal post type.
 */
add_action( 'init', 'wpse_register_post_type_proposal' );
function wpse_register_post_type_proposal() {
    $args = [
            'label'             => __( 'Proposals', 'textdomain' ),
            'public'             => true,
            'publicly_queryable' => true,
            'show_ui'            => true,
            'show_in_menu'       => true,
            'query_var'          => true,
            'rewrite'            => [ 'slug' => 'proposal' ],
            'capability_type'    => 'post',
            'has_archive'        => true,
            'hierarchical'       => false,
            'menu_position'      => null,
            'supports'           => [ 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' ],
    ];
    register_post_type( 'proposal', $args );
}

and the meta box:

/**
 * Register meta box
 */
function wpse_register_meta_boxes() {
    add_meta_box( 'meta-box-id', __( 'My Meta Box', 'textdomain' ), 'wpse_metabox_callback', 'proposal' );
}
add_action( 'add_meta_boxes', 'wpse_register_meta_boxes' );