customizer not working properly

Hi,
I’m trying to change the favicon and the header image of my site, but whatever I do, I keep getting the message “Seems like something went wrong. Wait a few seconds and try again.”
I’ve tried a few dozens of times, but nothing works.
I use Hueman 3.4.0 and WordPress 4.9.7.

Hope there is some help out there!

The page I need help with: [log in to see the link]