LE has a single hardcoded default user (root) and default but changeable password (libreelec) with SSH disabled by default. In LE settings you can enable SSH, and enable/disable SSH passwords (disabling allows you to stop password login and use pre-installed SSH private keys instead) and change the root user password. If all you want to do is login remotely .. re-enable SSH passwords and use any SSH client to access.
Once you have SSH access sorted, run "pastekodi" after booting and share the URL so we can see the Kodi log.