Thanks for the quick replies guys
Reversing channels didn't work. Setting the end points shouldn't be necessary, I only had to do it once for this transmitter. I tried it anyway, but couldn't get to the setup menu.
This brought me back to the power button, and it seems this has been the problem all along. I re-opened the casing of the power button and apparently I didn't see it last time: one single strand of copper wire was touching another type of wire. It's the white wire, probably the signal wire?
Whatever the case is, I took the strand out of the way and the throttle seems to work again!
Now this shows the exposed wiring is more of a problem than I thought, so I'll solve that soon!