JD-800 bender pot issue

So the bender pot on my JD-800 does not send any Frequency Modulation or vibrato anymore when pushed forward. The Pitch bending, up and down, seems to work fine. Sending modulation data over MIDI from another synth controller shows that the JD-800 does receive and playback the vibrato for the patch selected. So something is clearly wrong with the controller. Any ideas? Thank you in advance.