Hi @Holgerrpl. Not running a Thinkpad but want to mention one thing I learned about the TTY console modes and how they tie directly into the terminal mode grub sets up for itself. Things like grub displaying vs not displaying making a difference and settings in /etc/default/grub for grub graphics also setting the tty resolution.
It got me flustered trying to get a decent TTY resolution while grub was in text mode. It's something you might test:
It wouldn't surprise me if TTYs started working.