When I try to install something like a theme or window borders,then I get this error https://i.nuuls.com/LMHwi.png
That means there was a network error. Either the server is down, or your home network is flaky, or something like that. But it's underneath the software layer. FWIW it's working fine for me, and the fact that you see the old GHNS dialog means that you're not using the latest version of Plasma. I would recommend upgrading.