It's caused by (some) ad blockers because you call ga() (google analytics) in your javascript which loads through google tag manager which is blocked from loadi

It's caused by (some) ad blockers because you call ga() (google analytics) in your javascript which loads through google tag manager which is blocked from loading by default with uBlock for example. This all causing the modal window to not show, leading to a catch error forwarding to your 404 page.