Refused to apply style from... because its MIME type ('text/html') is not a supported style-sheet MIME type, and strict MIME checking is enabled

Stylesheets should have a MIME type of text/css. Your server is serving up a HTML document (probably a 404 NOT FOUND error page) when it should be serving a CSS document.

Solution:
In my case, the problem was an empty @GetMapping in one of my Controllers. I deleted it and the problem was gone.

I think the same answer will help someone who's having this similar problem and context ; e.g. JSP.

Background:
I had the same problem using Spring Boot and FreeMaker.