I just created a brand new installation of Flarum on my hosting.
I do not know if this is a bug or not, but I just encountered this error:
After searching on Google, I found out that this is because some files were not found on the server on some chinese website and I wanted to share this feedback with you.
In the folder: vendor/components/font-awesome/less you should have:
After copying these files over there, the error disappeared.