@inside_the_square i am seeing the same exact issue. in my case, i am trying to use the custom CSS to insert an SVG version of my logo in the header. i'm sure my syntax or something is wrong as i'm not very experienced with CSS. it must be something simple. here's what i have:
.site-title img { width: auto; height: auto; background-image: url(/s/Sentient_Logo_stacked-2.svg); }
this was after trying lots of different versions without success. not it works on the CSS editor page, but not on the live site.