[seyfin]

I believe the issue is related to your custom skin that is not compatible with the new X-Cart version 5.2.6. Try to disable the skin module in your X-Cart admin back-end to verify my guess.

If that is the case, you will need your custom skin adopted to be compatible with the new X-Cart version 5.2.6. Specifically, for your customer skin, you will need to develop getLayoutTypes() method for class 'XLite\Module\Pmall\PmallSkin\Main', in order to fix the errors, for example:

    public static function getLayoutTypes()
    {
        return \XLite\Core\Layout::getInstance()->getLayoutTypes();
    }

See also:
* X-Cart 5 Developer docs