Thanks for the help again: I've finally solved the problem without exactly knowing what was wrong.
After my last bash session I changed the cut-of voltage to 12v and didn't change anything. But the "write" to the ESC probably went bad and something got messed up.
When I got back home and used the castle link again, I started with a reset to defaults. After "writing" those defaults, I loaded it again and set the cut-off to 12v.
Success.
The truck now works flawlessly, and even seems to have more punch and top-speed. Can the updated firmware do that?