I fixed the problem. In fact the error is in the file account_manage.php in the line:
<td class="main" width="100%"><?php echo tep_draw_textarea_field('products_description[' . $languages[$i]['id'] . ']', 'soft', '20', '10', (isset($products_description[$languages[$i]['id']]) ? $products_description[$languages[$i]['id']] : tep_get_products_description($pInfo->products_id, $languages[$i]['id']))); ?></td>
While in the admin/cathegories.php it is written:
<td class="main"><?php echo tep_draw_textarea_field('products_description[' . $languages[$i]['id'] . ']', 'soft', '70', '15', (isset($products_description[$languages[$i]['id']]) ? ereg_replace('& ', '& ', trim(stripslashes($products_description[$languages[$i]['id']]))) : tep_get_products_description($pInfo->products_id, $languages[$i]['id']))); ?></td>
The good line is the admin/cathegories.php line.
That's all.