Mac Mini Server updated to OSX Server 10.6.7 can't log in

Hello,

This afternoon I updated my Mac Mini Server 2010 to Mac OS X Server 10.6.7 (not the combo version as the update was about 700Mb).

Immediately after rebooting, I could no longer log in at the Open Session prompt. The right combination of login/password always leads to the window shaking left and right to say the login information is wrong.

Problem is, the login info is correct, as I can log onto the server using SSH.

I tried resetting the password with sudo passwd myusername, but it did not change anything. I tried a different password, each time it works with SSH but not with the session opening panel.

Do you have any idea where this comes from? How can I revert to 10.6.6 if necessary?