Episerver 8 and 9 are configured to use the nuget feed URL https://nugetaddons.episerver.com/feed/Addons.svc . That site has a license error, which is causing the feed XML to be invalid, and in turn make the Add-On Store feeds return "no add-ons available". (URL found in Episerver.Packaging assembly v 220.127.116.11 and v. 18.104.22.168)
Steps to fix: Install a new license on the nugetaddons.episerver.com site.
For details, see the "Troubleshooting" section at the bottom of my blog post: https://www.epinova.no/en/blog/configuring-the-add-on-store-in-episerver-7-8-and-9/
Question: Why aren't the default Add-On Store feeds configured to use the official nuget feed at http://nuget.episerver.com/feed/packages.svc/ ?