Okay, I installed a new clean “contact” template from your theme and it still crashes IE11.
I also removed my child theme mods and text mods to the original contact form with no changes.
So, you’ve got a clean contact form that crashes after a successful submittal when trying to navigate away from the contact page while staying on the same site.
Maybe you fixed this in your release this morning, if so please let me know.
At this point I am going to go with a generic Jetpack contact form as I do not like the theme one crashing the browser. So what you will see onsite is not your “contact” template.