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.
Thank you for bringing this to our attention. You are 100% correct and we fixed the problem
That response sounds an awful lot like customer service at a major ISP, or automatic email replies in helpdesk systems.
Sorry for not coming up with a more original response- but thats the truth :)
On our side, everything is working ok, and the site is running well, so our tech guys have to investigate why a user experienced an error