“error in your SQL syntax”

Right, so this occurs when adding a new product and is from includes/Frontend/Product.php‘s call of Group::getByProduct( $product, true ); near line 439 in function wc_tab_addons.

It’s the only time you call includes/Model/Group.php‘s function getByProduct with true as second parameter ($single), but you never observe the latter’s value, instead $single is unconditionally overwritten near line 327…
$single = get_post_meta($id, '_zaddon_disable_global', true) === "yes";
… and that’s always false for entirely new product objects?

So seems like you forgot that $single ought to be ||‘ed with itself (or |=‘ed rather than =‘ed) in that assignment?

There is yet another path this same error will occur: If I open a category site in Divi Theme Builder, I see it triggered six times in addition to the actual products listed (for which it runs fine). The call path is:

1. includes/Frontend/Shop.php: add_to_cart_text calls Frontend::hasTypes( $product )
2. includes/Frontend.php: hasTypes calls Group::getByProduct($product)

Right, so rather than guessing around for ages & praying there aren’t even more bugged paths, I just rewrote the part in getByProduct to

1. actually observe $single as parameter
2. use the $single query if $categories is empty

Dunno if that’s as intended but it can’t be worse than the bugs, which lead to no query being executed at all.

The section of code is now:

	$query = "
		SELECT
			g.* , GROUP_CONCAT(DISTINCT c.category_id) as <code>categories</code>, GROUP_CONCAT(DISTINCT p.product_id) as <code>products</code>
		FROM {$table} g
			INNER JOIN {$p2g} AS p ON (p.group_id = g.id)
			LEFT JOIN {$p2g} pf ON (pf.group_id = g.id AND p.product_id <> pf.product_id)
			LEFT JOIN {$c2g} c ON (c.group_id = g.id)
		WHERE pf.product_id IS NULL AND c.group_id IS NULL AND p.product_id IN {$related_ids}
		GROUP BY g.id
		ORDER BY g.priority ASC
		";

        if (!($single || get_post_meta($id, '_zaddon_disable_global', true) === "yes")) {
            $product = $product->is_type('variation') ? wc_get_product($product->get_parent_id()) : $product;
            $categories = $product->get_category_ids();
            if ($categories) {
                $categories = array_map('intval', $categories);
                $categories = '(' . implode(',', $categories) . ')';
                $query = "
		SELECT g.*, GROUP_CONCAT(DISTINCT c.category_id) as <code>categories</code>, GROUP_CONCAT(DISTINCT p.product_id) as <code>products</code>
			FROM {$table} as g
		LEFT JOIN {$c2g} as c ON c.group_id = g.id
		LEFT JOIN {$p2g} as p ON p.group_id = g.id
			WHERE c.category_id IN {$categories} OR p.product_id IN {$related_ids} OR g.apply_to = 'all'
		GROUP BY g.id
		ORDER BY g.priority ASC
		";
            }
        }