when it comes to web games, there is a thing called browser cache that keeps copies of already downloaded files from the web server. so if you update your game and player has this issue, while he played your game before the update, he will get this error, because he has cached old files. usually what solves the problem is to add some postfix to every file name (or fire file request with query param with build number) so browser will no longer think that has this file in cache.