Update: Just want to inform you that this was not a problem before...

By any chance, are you using Siteground's staging site feature? Because I do suspect that the problem here is the way Siteground is actually rewriting the urls. They are doing this on the webserver level and because our javascript files are cached (written on the file system), we retrieve the site url from a cached file. Hence, it always tries to make an ajax request to http://xxxxxxx.com/ rather than http://staging.xxxxx.com which is a domino effect and cause the rest of the javascript codes to fail altogether.

This my opinion. May I know if you get any error during the update? Or you can provide us your backend access to check whether this issue is related or not with what I mentioned above.