Hey there, I hope you are well.
It sounds like there may be a version difference between your backup and the new Discourse instance, especially since the original instance was last updated in May 2021.
To resolve this, try matching the versions by checking the old version in your app.yml file, switching the new instance to that version, and then rebuilding. If that doesn’t work, reset the settings and clear any cache by running Rails.cache.clear inside the container and then rebuilding the app.
This should help restore access to your admin pages and ensure everything is compatible.
Thank You,
Mike Taku.