OK, this was quite interesting... I've never seen this before and without FTP access, I probably would've pinpointed it sometime in summer 2025...
The culprit was your includes/languages/english/responsive_classic/index.php file - and to be perfectly honest, I'm still scratching my head about it... I have no idea what exactly caused it, but I replaced that file with a fresh version from includes/languages/english/index.php (and removed the 2 lines like in your original file) and it's working.
What's even more interesting is that the file shows as identical, but I noticed the header was broken on product listing pages as well, but was fine on product_info and shopping_cart page, which pointed me in the direction of index page... Interesting, I must admit.
As a side note - I've noticed you're using ZC 1.5.6c, but some of your files (functions_prices.php, categories_ul_generator.php, split_page_results.php etc) are from much older versions of Zen Cart. You might wanna consider fixing that because it might cause problems in the future.