Can't install Eclipse plug-in

Hey, this seems like it's a fairly common error, but I can't get past it. I've a clean install of Eclipse 4.14 on a Mac running Catalina OS (10.15.2) and if I try to install the plugin, I get

Unable to read repository at developer.garmin.com/.../.

Unable to read repository at developer.garmin.com/.../.

developer.garmin.com/.../ is not a valid repository location.

I'm at home, not behind any kind of firewall. The only clue I have is that I _was_ able to install the plugin on a previous installation of Eclipse, but I had to remove that one and reinstall

because it was the C/C++ flavor of Eclipse which I installed, and I needed to install the Java dev one.