Bug site [PrestaShopDatabaseException]

Bonjour,
Du jour au lendemain , notre site (passage d'un serveur mutualisé à un serveur dédié il y a 3 mois) affiche l'erreur ( cfci dessous). Cela est apparu comme cela, sans aucune modif ou intervention de notre part. prestashop 1.6.
Je cherche un prestataire pour débuger cela très rapidement. merci

[PrestaShopDatabaseException]

Can't create/write to file '/tmp/#sql_428_0.MYI' (Errcode: 28)

SELECT cp.`id_product_attribute`, cp.`id_product`, cp.`quantity` AS cart_quantity, cp.id_shop, pl.`name`, p.`is_virtual`,
pl.`description_short`, pl.`available_now`, pl.`available_later`, product_shop.`id_category_default`, p.`id_supplier`,
p.`id_manufacturer`, product_shop.`on_sale`, product_shop.`ecotax`, product_shop.`additional_shipping_cost`,
product_shop.`available_for_order`, product_shop.`price`, product_shop.`active`, product_shop.`unity`, product_shop.`unit_price_ratio`,
stock.`quantity` AS quantity_available, p.`width`, p.`height`, p.`depth`, stock.`out_of_stock`, p.`weight`,
p.`date_add`, p.`date_upd`, IFNULL(stock.quantity, 0) as quantity, pl.`link_rewrite`, cl.`link_rewrite` AS category,
CONCAT(LPAD(cp.`id_product`, 10, 0), LPAD(IFNULL(cp.`id_product_attribute`, 0), 10, 0), IFNULL(cp.`id_address_delivery`, 0)) AS unique_id, cp.id_address_delivery,
product_shop.advanced_stock_management, ps.product_supplier_reference supplier_reference, IFNULL(sp.`reduction_type`, 0) AS reduction_type,
NULL AS customization_quantity, NULL AS id_customization,

product_attribute_shop.`price` AS price_attribute, product_attribute_shop.`ecotax` AS ecotax_attr,
IF (IFNULL(pa.`reference`, '') = '', p.`reference`, pa.`reference`) AS reference,
(p.`weight`+ pa.`weight`) weight_attribute,
IF (IFNULL(pa.`ean13`, '') = '', p.`ean13`, pa.`ean13`) AS ean13,
IF (IFNULL(pa.`upc`, '') = '', p.`upc`, pa.`upc`) AS upc,
pai.`id_image` as pai_id_image, il.`legend` as pai_legend,
IFNULL(product_attribute_shop.`minimal_quantity`, product_shop.`minimal_quantity`) as minimal_quantity,
IF(product_attribute_shop.wholesale_price > 0, product_attribute_shop.wholesale_price, product_shop.`wholesale_price`) wholesale_price

FROM `prefpres_cart_product` cp
LEFT JOIN `prefpres_product` `p` ON p.`id_product` = cp.`id_product`
INNER JOIN `prefpres_product_shop` product_shop ON (product_shop.`id_shop` = cp.`id_shop` AND product_shop.`id_product` = p.`id_product`)
LEFT JOIN `prefpres_product_lang` `pl` ON
p.`id_product` = pl.`id_product`
AND pl.`id_lang` = 2 AND pl.id_shop = cp.id_shop
LEFT JOIN `prefpres_category_lang` `cl` ON
product_shop.`id_category_default` = cl.`id_category`
AND cl.`id_lang` = 2 AND cl.id_shop = cp.id_shop
LEFT JOIN `prefpres_product_supplier` `ps` ON ps.`id_product` = cp.`id_product` AND ps.`id_product_attribute` = cp.`id_product_attribute` AND ps.`id_supplier` = p.`id_supplier`
LEFT JOIN `prefpres_specific_price` `sp` ON sp.`id_product` = cp.`id_product`
LEFT
JOIN prefpres_stock_available stock
ON (stock.id_product = cp.id_product AND stock.id_product_attribute = IFNULL(`cp`.id_product_attribute, 0) AND stock.id_shop = 1 AND stock.id_shop_group = 0 )
LEFT JOIN `prefpres_product_attribute` `pa` ON pa.`id_product_attribute` = cp.`id_product_attribute`
LEFT JOIN `prefpres_product_attribute_shop` `product_attribute_shop` ON (product_attribute_shop.`id_shop` = cp.`id_shop` AND product_attribute_shop.`id_product_attribute` = pa.`id_product_attribute`)
LEFT JOIN `prefpres_product_attribute_image` `pai` ON pai.`id_product_attribute` = pa.`id_product_attribute`
LEFT JOIN `prefpres_image_lang` `il` ON il.`id_image` = pai.`id_image` AND il.`id_lang` = 2
WHERE (cp.`id_cart` = 36756) AND (p.`id_product` IS NOT NULL)
GROUP BY unique_id
ORDER BY cp.`date_add`, p.`id_product`, cp.`id_product_attribute` ASC

at line 646 in file classes/db/Db.php

641. WebserviceRequest::getInstance()->setError(500, '[SQL Error] '.$this->getMsgError().'. From '.(isset($dbg[3]['class']) ? $dbg[3]['class'] : '').'->'.$dbg[3]['function'].'() Query was : '.$sql, 97);
642. }
643. elseif (_PS_DEBUG_SQL_ && $errno && !defined('PS_INSTALLATION_IN_PROGRESS'))
644. {
645. if ($sql)
646. throw new PrestaShopDatabaseException($this->getMsgError().'<br /><br /><pre>'.$sql.'</pre>');
647. throw new PrestaShopDatabaseException($this->getMsgError());
648. }
649. }
650.