I had this same problem and was going crazy trying to find a solution. My site would load fine, with all assets and scripts running perfectly; however, I would receive the pop-up error. Finally, I did a full export to my desktop into a new folder and when I opened this copy, it worked fine. I compared the two HTML files I had (my original and my newly exported), the problem was with the auto-generated numbers that muse tacks on to the CSS files:
<link rel="stylesheet" type="text/css" href="css/index.css?442368484"
My site would load fine in either version, but for whatever reason the new export had different numbers. Once I changed the numbers in my original to match, I stopped receiving this error!