@jvalleroy, initial report contains the ouput as id as:
uid=10000(foo) gid=100(users) groups=100(users),118(_tor-plinth)
So, the user is somehow not part of the admin group but instead part of _tor-plinth group. The latter group is created when Tor is setup and FreedomBox creates a tor instance called ‘plinth’. foo should not be part of that group for any reason. At the same time foo user should have been part of admin group and is not. foo user is likely the first user created during initial setup given the UID being 10000. Strangely, this situation occurred after a bit of usage.
@kat85, are you able to login to FreedomBox web interface? Does the interface show you as administrator (you will be able to see the apps and system pages). If so, you can try to create a new user with administrator privileges and see if that user is able to do sudo and has proper information for id user2. Also could you post the output of getent group? Thanks for your patience with the issue.