There is an official bug tracker for Rufus.
If you think you have an issue with the Rufus application, can you please report it there? Also, when you do, please make sure you provide the log(s) from Rufus.
For the record, I am the Rufus developer and, I am not seeing any download issue in Rufus (or reports of download issues) with the latest Ubuntu 18.10 ISO, so I strongly suspect the issue is with your environment. Especially, if your system's SSL libraries are not up to date (which is a common issue on Windows 7, less so on Windows 8 or later), you may not be able to download the files over HTTPS (whereas Rufus 2.18 downloads the files over HTTP).
Furthermore, considering that the HTTP URLs that Rufus 2.18 used to download files are no longer valid, I suspect that the reason why you find that Rufus 2.18 "works" is simply because you used that version before the HTTP download links were removed, and a copy of the required files still exist in the
rufus_files\ folder you should see in the same location as the Rufus application. Either that or you chose to create the drive in GPT/UEFI mode in 2.18, in which cases the files don't need to be downloaded.
Either way, I'm afraid it's just impossible to address your issue unless your report it through the proper channels and provide a full log of what you did, so that we can see why the downloads failed.