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J'ai identifié le fichier qui cause à priori le bug sous PHP 7.4 (pas de soucis sous PHP 7.3) (j'ai remplacé mes fichiers du thème un par un jusqu'à trouver celui qui déclenche l'erreur en mode debug), voici sa composition (cart-detailed-totals.tpl) :

{**
													  
 * 2007-2017 PrestaShop
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Academic Free License 3.0 (AFL-3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * https://opensource.org/licenses/AFL-3.0
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to [email protected] so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade PrestaShop to newer
 * versions in the future. If you wish to customize PrestaShop for your
 * needs please refer to http://www.prestashop.com for more information.
 *
 * @author    PrestaShop SA <contact@prestashop.com>
 * @copyright 2007-2017 PrestaShop SA
 * @license   https://opensource.org/licenses/AFL-3.0 Academic Free License 3.0 (AFL-3.0)
 * International Registered Trademark & Property of PrestaShop SA
 *}
{block name='cart_detailed_totals'}
<div class="cart-detailed-totals">

  <div class="">
    {foreach from=$cart.subtotals item="subtotal"}
      {if $subtotal.value && $subtotal.type !== 'tax'}
        <div class="cart-summary-line" id="cart-subtotal-{$subtotal.type}">
          <span class="label{if 'products' === $subtotal.type} js-subtotal{/if}">
            {if 'products' == $subtotal.type}
              {$cart.summary_string}
            {else}
              {$subtotal.label}
            {/if}
          </span>
          <div>
          <span class="value">
              {if 'discount' == $subtotal.type}-&nbsp;{/if}{$subtotal.value}
          </span>
          {if $subtotal.type === 'shipping'}
              <small class="value">{hook h='displayCheckoutSubtotalDetails' subtotal=$subtotal}</small>
          {/if}
          </div>
        </div>
      {/if}
    {/foreach}
  </div>

  {block name='cart_summary_totals'}
      {include file='checkout/_partials/cart-summary-totals.tpl' cart=$cart}
  {/block}
  <hr class="separator mt-0">

    {block name='cart_voucher'}
    {include file='checkout/_partials/cart-voucher.tpl'}
  {/block}
</div>

{/block}

Si quelqu'un saurait me dire si l'incompatibilité avec PHP 7.4 vient d'un élément de ce fichier, merci beaucoup, si je retire ce passage, je n'ai plus d'erreur sur ma page panier : 
 

{foreach from=$cart.subtotals item="subtotal"}
      {if $subtotal.value && $subtotal.type !== 'tax'}
        <div class="cart-summary-line" id="cart-subtotal-{$subtotal.type}">
          <span class="label{if 'products' === $subtotal.type} js-subtotal{/if}">
            {if 'products' == $subtotal.type}
              {$cart.summary_string}
            {else}
              {$subtotal.label}
            {/if}
          </span>
          <div>
          <span class="value">
              {if 'discount' == $subtotal.type}-&nbsp;{/if}{$subtotal.value}
          </span>
          {if $subtotal.type === 'shipping'}
              <small class="value">{hook h='displayCheckoutSubtotalDetails' subtotal=$subtotal}</small>
          {/if}
          </div>
        </div>
      {/if}
    {/foreach}

Merci