server - Cannot login using correct password

I installed Ubuntu server on a PC, with correct user + password.

We installed webmin, and we are running a website on this PC. It works. I can remotely update webmin and ubuntu package.

However, I am facing a serious issue: I cannot log in on a session using user + password.

What happens here: when entering the correct password, the session intends to open for 1sec, screen becomes black as if I enter in the session, but it comes back to the user/password menu, as if my password was wrong.

But I confirm that my password is correctly input as well as user name.

First I though that we made a mistake during installation by defining a wrong password, because of keyboard definition (as we are in Japan), but it seems that the problem is not this one.

So it is still impossible to enter in any session excepted guest session.

However, even I tried to login as admin or root, it does not work too.

So the web site works! I can update remotely ubuntu, but I cannot enter in the session when I am in front of the computer! It is really difficult for me to understand and to recover this strange problem.

Any idea will be really helpful!