Ok @romanhruska I think I have a possible explanation.
Something in your instance is forcing an https prefix for an SVG namespace constant. When I inspect the JS code, the equivalent for this line in core returns https://www.w3.org/2000/svg where as you can see in core, it should be http://www.w3.org/2000/svg. (I also tested locally, edited that line to use https, and I have a perfect repro.)
Do you have any patches that do this? If not, does your server do some magic to ensure all http strings in JS are turned into https?