The solution to this was highlighted to the developers four months ago and we are still waiting for them to acknowledge it:
https://www.ads-software.com/support/topic/recent-update-causing-php-error/#post-13223852
In other issues they mark it as “resolved”:
https://www.ads-software.com/support/topic/media_buttons_context-deprecated-2/
The log itself holds the answer:
PHP Deprecated: media_buttons_context is <strong>deprecated</strong> since version 3.5.0! Use media_buttons instead. in /path/to/wp-includes/functions.php on line 5234
Stacktrace:
/path/to/wp-includes/plugin.php(638): _deprecated_hook('media_buttons_c...', '3.5.0', 'media_buttons', '')
/path/to/wp-admin/includes/media.php(656): apply_filters_deprecated('media_buttons_c...', Array, '3.5.0', 'media_buttons')
/path/to/wp-includes/class-wp-hook.php(287): media_buttons('acf_content')
/path/to/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array)
/path/to/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
/path/to/wp-includes/class-wp-editor.php(235): do_action('media_buttons', 'acf_content')
/path/to/wp-includes/general-template.php(3536): _WP_Editors::editor('', 'acf_content', Array)
/path/to/wp-content/plugins/advanced-custom-fields-pro/includes/assets.php(521): wp_editor('', 'acf_content')
/path/to/wp-includes/class-wp-hook.php(287): ACF_Assets->print_uploader_scripts('')
/path/to/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
/path/to/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
/path/to/wp-admin/admin-footer.php(78): do_action('admin_footer', '')
/path/to/wp-admin/post.php(369): require_once('/home/customer/...')
{main}