Sometimes my KDE freezes but I can switch to TTY using Ctrl-Alt-F2
but I wasn’t able to login with user “root” and password.
Should this be possible?
No.
The login is for your username and password that you use when logging
in to Qubes. Once logged in as your username, you can use sudo to
do root operations.
Mike.
I wasn’t even aware that I had an username there 
In Qubes it’s always just “user” and when I log in after restarting the machine I can’t remember to ever need to enter a username, just the password.
User is probably already autofilled from previous logins.
But it worked. I tried with the username I usually use and I could log in.
Thanks!