I just had the same issue with Font Awesome. Turned out this was caused by a problem with FTP. The file was uploaded as text (ASCII) instead of binary, which corrupted the file. I simply changed my FTP software to binary, re-uploaded the font files, and then it all worked.
https://css-tricks.com/forums/topic/custom-fonts-returns-failed-to-decode-downloaded-font/ this helped me in the end I had the same issue with FTP transferring files as text