@frikic Sorry to hear you were having troubles, glad to hear you've found a solution. Please note that the spine-unity 4.1 runtime does not officially support Unity 6.
I've tried the above package manager URL
https://github.com/EsotericSoftware/spine-runtimes.git?path=spine-unity/Assets/Spine#4.1
with Add package from git URL in Unity 6000.0.37 without any issues. While the error message hints at some other problem (network and firewall related, or perhaps directory write permissions): did you add the spine-csharp package first before adding the spine-unity package? Did that work without issues?
Regarding formatting here on the forum: Please note that you can enclose text or URLs like the ones above in single backticks "`" to format them as inline code, and triple-backticks "```" for multi-line code. Otherwise they are a bit hard to read 🙂.
