magento2.3 – Magento 2.3 migration problem, configurable products that cause error 503

I am faced with these problems after migration of data from Magento 1.9.4 to Magento 2.3:

  • the configurable products are causing 503 server errors in the front-end.
  • The categories with configurable products are causing 503 server errors.
  • the search for a configurable product is causing 503 server error.

  • PHP LOGS

    [21-Mar-2019 03:16:01]    WARNING: [pool example.com] child 57516 left signal 11 (SIGSEGV) after 1,659686 seconds from the start
    [21-Mar-2019 03:16:01] REALIZE: [pool example.com] child 57576 started
    
  • DOMAIN REGISTRIES:

    AH01067: Error reading the FastCGI header, reference: https://example.com/

    (104) Restore the connection by pair: AH01075: Error sending the request to :, referent: https://example.com/

  • The products have unique URLs

  • I managed to work by opening and saving a random product.

• Magento LOGS:

main.ERROR: Warning: SessionHandler :: read (): open (/ var / lib / php / session / sess_ub6spo4jcfmsh0itec4jahoik8, O_RDWR) failed: Permission Denied (13) in /var/www/vhosts/example.com/httpdocs/vendor /magento/framework/Session/SaveHandler/Native.php on line 22

Ambient :
Ubuntu 18.04.2 LTS
Plesk Onyx – Version 17.8.11 Update # 45
PHP 7.2.16 – FPM application served by Apache
Nginx: proxy mode + intelligent processing of static files

PHP configuration:
max_input_vars = 10000
max_input_nesting_level = 800
precision = 14
output_buffering = 44096
Memory limit: 10GO
max_execution_time: 180000
max_input_time: 180000