Cannot save theme – Failed to load sprite in Builder

Hi, suddenly all my websites cannot save the theme in Builder. In all of them I get a pop-up background which prevents me from making a selection whether the theme should be activated or not. And that's it, it freezes like that. See screenshot below.

I also found out what is the problem: Builder is incorrectly referencing the image:

It expects it to be in []/themes/upfront/img/uf-ui-sprite.svg, in reality everything would be in []/wp-content/themes/upfront/img/uf-ui-sprite.svg, as you can see in this screenshot:

Please fix or tell me how to fix as I cannot do anything at the moment in any of my websites.