First Admin account not showing on login page

I created a second Admin account on my Macbook Pro (Yosemite) and also enabled Guest login. Now I can only access the second account and the guest login from the login screen, the primary admin account is no longer shown.

It is also not shown in the fast user switching menu, nor am I able to log into it when I select username and password as login option.

I have not activated FileVault and I have not hidden the user by adding it to the HiddenUsersList. Any other things I might check?

I am able to su to the old user and the home directory and all still exist.