I’ve downloaded and installed OSMC on a new Raspberry Pi 2
It appears that the Isengard version of Kodi in the latest OSMC is looking at http://mirrors.kodi.tv/addons/gotham/addons.xml for the official addon repository… but that link simply 404’s.
I suspect that the correct URL should be http://mirrors.kodi.tv/addons/isengard/addons.xml which works.
Do you know when this bug was introduced so I can get a build that isn’t broken?