SUMMARY
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I believe I might have accidentally stumbled upon why KDE Connect so often does not connect or constantly disconnects between Windows 10 and Android. I believe it might have something to do with a foobar 2000 conflict.
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I've been dogging this problem for about a year and almost completely given up on using KDE Connect even though it is useful when it works. I've downloaded/installed just about every Windows version there is from https://download.kde.org/Attic/release-service/ and most of the Android versions from https://www.apkmirror.com/apk/kde-community/kde-connect. I'm currently using builds Android v 1.29.0 and Windows kdeconnect-kde-21.12.3-903-windows-msvc2019_64-cl as they seem to be the "most" stable (even though that's relative). Yes, I've tried the current kdeconnect-kde-23.08.1-1463-windows-cl-msvc2019-x86_64, both downloaded from the Windows Store and directly from https://kdeconnect.kde.org/download.html (as well as the most recent Android builds). They all constantly disconnect (if they even connect at all), and seemingly without clear cause. I've deleted/recreated Windows firewall rules to no avail, and obviously I've tried to apply https://userbase.kde.org/KDEConnect#Troubleshooting. Rebooting Windows sometimes helps enable the connection (not just killing/restarting KDEConnect), but not always.

I just stumbled upon two oddities: first, when I close foobar2000 and then Find Devices on KDEConnect, devices are found and they connect. However, as soon as I reopen foobar2000, the KDEConnect connection to the devices die. I've uninstalled various foobar2000 plugins/components which seem like they might cause a conflict (e.g., UPnP MediaRenderer Output.fb2k-component, foo_upnp.fb2k-component) but it doesn't make a difference. It also doesn't seem to matter which KDEConnect plugins are enabled.

Second, sometimes KDEConnect will connect properly while foobar2000 is running. For the life of me, I can't determine why. I'm wondering if there are port conflicts because KDEConnect is constantly using different ports but, at least on Windows, I can find no way of creating a KDEConnect log that shows the potential port conflict (or any other errors for that matter). foobar2000's log also doesn't reveal anything.

What is clear is that foobar2000 works always - no matter if KDEConnect is running or not. However, KDEConnect only sometimes works when foobar2000 is running. From my research, it does not appear you can change the order/priority of firewall rules in Windows (e.g., I can't make the KDEConnect allow rules higher priority than foobar2000's rules).

Any ideas how to troubleshoot further? I'd love to help you solve this. There are so many posts all over the internet where people have given up trying to get Windows KDEConnect to work and I'm wondering if it's all because they also are running foobar2000. Thanks.

SOFTWARE/OS VERSIONS
Windows 10 (KDEConnect build 23.08.1-1463)
Android 14 (KDEConnect build 1.29.0)
foobar 2.1.1 (32-bit)