If you get the following error message when launching the Android SDK Manager:
Failed to fetch URL https://dl.google.com/android/repository/addons_list-2.xml, reason: File not found
most likely the owner/permissions are not set correctly for the ~/.android directory.
This error message is very confusing and it took me while to find the issue.
In my case the owner of this directory and all the files and directories in it was root.
There are numerous “solutions” given to this in many forums. Most of them recommend launching Android Studio as root. That is definitely the wrong approach. It will get rid of the error, since you will no longer be faced with permissions issues, but running Android Studio as root is a vary bad idea.
Instead, you should change the owner of the ~/.android directory to your user id.
So, find the .android directory (most likely it is in your home directory). Then change the ownership recursively to your user:
cd ~ sudo chown -R myuser: .android/
You should be now good to go.