Thank you so much for this suggestion. When I disable ModSecurity for the domain, the editing issue does indeed goes away and the problem is fixed. I will reach out to the host for a fix.

You can certainly try activating it yourself so long as you use the FTP connection to restore the old theme afterwards.

I just tried updating to the new version 4.8.6 and got the same error as before:

[03-Aug-2021 20:12:01 UTC] PHP Fatal error: Uncaught Error: Call to undefined method avia_magazine::get_defaults() in /home/XXXX/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/magazine/magazine.php:526
Stack trace:
#0 /home/XXXX/public_html/wp-content/themes/enfold/config-templatebuilder/avia-template-builder/php/class-shortcode-template.php(1258): avia_sc_magazine->shortcode_handler(Array, ”, ‘av_magazine’, Array)
#1 /home/XXXX/public_html/wp-includes/shortcodes.php(356): aviaShortcodeTemplate->shortcode_handler_prepare(Array, ”, ‘av_magazine’)
#2 [internal function]: do_shortcode_tag(Array)
#3 /home/XXXX/public_html/wp-includes/shortcodes.php(228): preg_replace_callback(‘/\\[(\\[?)(av_hea…’, ‘do_shortcode_ta…’, ‘\n[av_hr class=’…’)
#4 /home/XXXX/public_html/wp-content/themes/enfold/config-templatebuilder/avia-template-builder/php/class-shortcode-helper.php(506): do_shortcode(‘\n[av_hr class=’…’)
#5 /home/XXXX/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/colu in /home/XXXX/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/magazine/magazine.php on line 526