Resolved
Add-on installation for all users is now fully functional again.
The problem was a faulty migration in our add-ons system, which has been corrected.
Issue
Add-ons installation is currently offline. Users are currently not able to install new add-ons from the command line or the web.
Installed add-ons are not affected by this issue, and continue to function normally.
We have identified the problem, and expect to deploy a solution shortly.