Mac OS Sierra : passwords not working; cannot log in

Hi,

On my Macbook Pro 15" mid 2012 I tried the upgrade from El Capitan to Sierra a week ago. Upgrade went well, no errors or hick-ups. When getting to the login-screen after the final boot I could select my user (I have both a personal user and a Guest user), but the password would not take. It would give my a spinning ball for a couple of seconds, after which the screen would "shake" as if the password was not correct. I tried different keyboard layouts/changing upper and lower case/nothing worked. Also getting back to the "user selection" screen did not work (spinning ball and getting back to the password screen). I tried rebooting from cold, same result.

This was not fun. I got out my toolkit and entered Recovery mode. I performed following steps:

1. Re-installing Sierra about 10 times: same result

2. Changing password with "resetpassword" from terminal. Also no effect.

3. Trying to make a new user from terminal (did not even work, maybe sierra changed as respect to el capitan?)

4. System-check: no problems

Found some problems like this on the fora, but nothing exactly the same (most problems where with the keychain, but AFTER initial login). In my case the system just on't get me in

In the end I gave up and put back my backup from Time Machine (El Capitan). I did try it again, but with the same results. Must be some permission thing in the users that messed up, but since the "repair permission" button is greyed-out since a version or 2 ago, I have no idea how to solve it.

Anybody encountered this problem before? Or should I just wait for 12.1 and pray that somebody got it out?

Thanks