I tried top disable the password of my Debian user and must have done something stupid since now I can't login at all anymore with that user.
I do still have the root/sudo password. I tried to type in "root" as the user at the logon screen after the computer boots, but it didn't seem to accept that.
Is there a way to login to my Debian installation knowing only the root account password? Unfortunately I have not enabled SSH on the machine.
If there really is a root password enabled for your installation, then logging with with username root and the root password should work. (A sudo password is not a root password.) Make sure you're using a full screen "terminal"-style login for this, and not the GUI, because some graphical login managers may prohibit root logins.
If it doesn't, you will need to boot from a rescue system (your Debian installation USB, for example) and follow other instructions to reset or re-enable your user's account through a "chroot". For example, this answer for a similar question in Ubuntu, Resetting root password in Ubuntu
rootuser) or the sudo password (the password for your use who was added to sudoers)?sudo? Then yes, that is your user's password and not root's and you will need to boot from a rescue system as described in roaima's answer.