Hello again,
okay, please have a look at the attached screenshots. I have a full video of it if you are interested. I ran your Web Install and I am thousand miles away, located on another continent. There was not any problem - SetupBuilder installed (from the web) all your files successfully.
That means, your server was not under heavy load and was able to handle all requests. There was no "source file not found" / "404" error. Your product is ~400 MB, so perhaps your server can't handle the traffic if multiple users are downloading concurrently?
By the way, after reviewing your installation scenario, please allow me one question: why are you using Web Installation at all? As far as I can see, you always install all files. End users frequently do not need all the files contained in an installer (e.g. for updates). The Web Installation technology simplifies the user experience by first downloading a small "smart" installer which can retrieve just the pieces needed for the user's specific requirements. But if you always install all files, why don't you simply create a single-file setup.exe? I really do not see any advantage by using Web Installation in your specific case. Perhaps I am missing something.
And to work around your server issue and to stop the "source file not found" problem immediately, just unmark the "Enable Web Installation" feature in your SB project and recompile. SetupBuilder will generate a single-file installer with the same .exe name. Upload it to your site and you are done.
Friedrich