We can’t add this item to your shopping cart right now | cache file_locking magento 2.1.xx

When we want to add something to the shopping cart from the product detail page we get this message :

We are currently unable to add this item to your cart. (We can't add this item to your shopping cart right now)

Problem what is in exeption.log is :

 main.CRITICAL: Exception: Notice: Undefined index: file_locking in web/vendor/magento/zendframework1/library/Zend/Cache/Backend/File.php on line 992 ....

The problem is caused in this file :

vendor/magento/sendingframework1/library/Zend/Cache/Backend/File.php

In a debug session, the file blocked the cache because it contained a variable that was not fully acknowledged. On one server we had never had this problem but on another this came up all of a sudden .

Solution :

Change this

 protected $_options = array(
        'cache_dir' => null,
        'read_control' => true,
        'read_control_type' => 'crc32',
        'hashed_directory_level' => 0,
        'hashed_directory_perm' => 0700,
        'file_name_prefix' => 'zend_cache',
        'cache_file_perm' => 0600,
        'metadatas_array_max_size' => 100
    );

to :

 protected $_options = array(
        'cache_dir' => null,
        'file_locking' => false,
        'read_control' => true,
        'read_control_type' => 'crc32',
        'hashed_directory_level' => 0,
        'hashed_directory_perm' => 0700,
        'file_name_prefix' => 'zend_cache',
        'cache_file_perm' => 0600,
        'metadatas_array_max_size' => 100
    );

After this, all problems were solved and all products could be neatly added to the shopping cart.