I actually found the solution.
Because I don't want the users to see a <own user> folder, I took away the option of that, in the user settings in the Server (software), and since I always log in myself with admin to do work with stuff, I didn't feel like I needed one either - but taking away that makes the user into just a <sharing> user - and it does so also for the admin user - meaning that it locks out the password working in <system settings> - <users and groups> and being able to <lock up the option to to changes> If I had turned the mac server/mac mini off, for a restart - the password of logging into the admin user wouldn't work, and you are locked out.
Solution was that, to make the password start working again - I had to set the setting in the Server (software) back to <having a home folder>, making the admin user able to work with its password again.
Happy I did not restart this time, like I did the first time, and ended up with the problem.
Not the best, that in the software, you can make the computer not have a administrator profile/user left in the os - without getting a warning about it - and then suddenly not be able to get logged in again.
But, thanks.
Sorry my english, I am Norwegian.