Can't invite a particular user to a GitHub repo

I have a user that I want to invite to a private GitHub repository.

Any attempt inviting them (either via the API or the GitHub UI) fails with the following error: “User could not be added”.

I tried it on different repositories and it’s always the same error.

Also, I verified that I was able to invite other users to those same repos.

So it’s only failing to that particular user.

What am I doing wrong?