It sends like your server isn’t serving DDS files as images (see thread here: Skybox not rendered or missed in web download of publish)
How to fix does depend on server and the level of access you have.
It’s a bit of the pain, but the right person should be able to configure this in 10 mins. Unfortunately it’s not my area of expertise.
Edit: I also forgot that you should setup the server to enable gzip compression to save on download size. Again that varies on the server/service. I would have thought that generally is enabled by default.
Edit2: Just check your server and it looks like gzip isn’t enabled for your server.