Hi Pennybad,
Thanks for the link, I see the error you mentioned, it is trying to load this resource:
https://muted.flatwhitedesign.pw/wp-content/themes/FlatWhiteParentOne-child-custom/css/font-awesome.css?ver=5.4.2. This is version 5.x of FontAwesome, which seems like it shipped with your theme. Maybe this error has always been showing, maybe it is new based on a theme update? Hard to say for sure.
Additionally to FontAwesome 5.x mentioned above, I see FontAwesome 4.x being loaded successfully on the page, and I see at least one of the icons on your site showing up properly. Ignoring the 5.x issue for now, it seems you may need to go back and re-select/re-save some of your icon selections in your admin area to get the rest working.
Going forward you should investigate why your site is trying to load two versions of FontAwesome, and pick one to standardize on. The two are not compatible with each other, and if you switch from 4 to 5, you will need to re-select and re-save icons wherever you used them, so choose wisely!
I hope this helps,
Matt Keys