Error 500 in posts, pages editor – after upgrade

Hi,
I upgraded WordPress, Enfold and all plugins to the recent versions. Unfortunately, I can’t edit created posts, pages, products, etc, because I have error 500.The error appears also with disabled plugins.

After you suggested:

I had PHP 5.6.x version. I asked server admin to upgrade the engine. Currently I’m running PHP 7.2.1.

Unfortunately error_log on the server doesn’t indicate the issue, however WordPress debug mode shows:

Fatal error: Uncaught Error: Call to undefined function Avia_Builder() in /home/homesquare/domains/homesquare.pl/public_html/files/themes/hs-parent/config-templatebuilder/avia-template-builder/php/meta-box.class.php:131 Stack trace: #0 /home/homesquare/domains/homesquare.pl/public_html/files/themes/hs-parent/config-templatebuilder/avia-template-builder/php/meta-box.class.php(61): MetaBoxBuilder->init_boxes() #1 /home/homesquare/domains/homesquare.pl/public_html/wp-includes/class-wp-hook.php(286): MetaBoxBuilder->setUp(”) #2 /home/homesquare/domains/homesquare.pl/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #3 /home/homesquare/domains/homesquare.pl/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /home/homesquare/domains/homesquare.pl/public_html/wp-admin/admin.php(332): do_action(‘load-post.php’) #5 /home/homesquare/domains/homesquare.pl/public_html/wp-admin/post.php(12): require_once(‘/home/homesquar…’) #6 {main} thrown in /home/homesquare/domains/homesquare.pl/public_html/files/themes/hs-parent/config-templatebuilder/avia-template-builder/php/meta-box.class.php on line 131

I can not edit and create articles and products. My site has been in place for 4 days. Is there any solution for this? Please help.

I apologize for replicating the topic but I have not received any answers for two days. For this reason, I do not know if the ticket was prematurely closed.