RoderickGI and CountryBumkin,
If it works from a browser or at least downloads the mpl playlist file, then something has to be getting in the way of MC. Antivirus is always my first choice of a suspect.
Are you both just using Windows Defender?
Are you both on Win10? Not the Insider edition?
RoderickGI, thanks for all the details. The multiple port thing is odd and could be a clue. I'd be surprised if it's MC doing that.
Can you both play Radio JRiver?
Well, it seems the solution for me was to change from the internal Chromium and external Chrome browser, to the internal and external Internet Explorer browser. Cloudplay now works, mostly, both inside MC and in a browser. This is unfortunate, because I almost exclusively use the Chrome browser now, and switched to the Chromium browser to fix issues in MC previously. Besides, Microsoft are putting their money on Edge, based on Chrome(!) and not IE anymore.
For comparison, I am on the latest Windows 10 version available to me as a normal user. No insider version. That's version 1803 (OS Build 17134.706). I run Norton Internet Security, Antivirus and Firewall.
Yes, I can play Radio JRiver, using the chromium engine in MC. Also using the IE browser engine, just tested.
Now as a test I turned off my Norton Smart Firewall and Auto-protect, then switched back to the Chromium engine in MC, and Cloudplay worked.
So it is some combination of the browser engine and security software that is causing the problem. But wait, there's more. When I turned Norton Auto-protect back on, Cloudplay still worked. When I turned the Smart Firewall back on, Cloudplay still worked. I need to reboot to see if that situation persists.
Cloudplay at user.jriver.com using Internet Explorer isn't perfect, but works, with some caveats.
Log in and navigation is fine, as far as I used it.
If I click the play icon next to a playlist, IE asks if I want to download or open the file. In the Chrome browser this obviously defaulted to download without asking, probably as a security measure. Because opening files directly from the internet can be a risk. I haven't looked at whether I can change a setting to allow mpl files to always be opened in either browser, yet. I would have expected it to be possible to form the URL for the link to always open a file, and not have to ask. This is common enough for PDF files, but I guess they are being opened in the protected environment of the browser.
Back to IE, when I click Open, I get a security warning message, as per the first attached image. As you can see, I can tell IE to always trust Cloudplay website files I open.
When I click Allow, MC comes to the foreground, with the playlist loaded in Playing Now, and playback commences.
Now I assume that it does that because my PC has a File Association for FLAC file, set to MC. So the FLAC files are being played by the associated application, MC. What happens when MC isn't installed on the PC that accesses Cloudplay on the web? I guess I haven't read up on the intended use of Cloudplay on the web. Does it always have to have MC installed, or at least an App that can open mpl playlists and play FLAC files? I thought it might just play in the browser if there was no such App, but now I'm not so sure.
Now if I click on the download icon next to a playlist, we hit some trouble. IE actually asks if I want to allow the website to open an App on my computer. See second image. When I select Allow, then I get the third image, and can't go any further. Selecting "Look for an app in the store" and clicking OK opens the Windows Store and searches for "localhost". Naturally, that is meaningless. The URL behind that download icon, which is as I mentioned above similar to "localhost:52199/MCWS/v1/SharedPlaylists?function=download&downloadtype=playlist&id=1882", is not structured correctly for use on the website.
So the Play icon can download or open a playlist, and hence it will be played in the associated application (i.e. MC), and the Download icon can't actually do anything.
CountryBumkin, sorry this doesn't help you much. Does your fourth computer use the IE engine in MC, which would explain why it works? It is still quite possible that router settings play a part, or there could be a combination of security software and browser settings that are at fault here. The trouble is, the default settings for both should be sufficient. Cloudplay shouldn't need special attention. I for one will be careful in loosening any security controls.