Just noticed I'm having a problem with my TT and FBV Express (not MK2).
All floorboard functions except for tap tempo work. Tap tempo via FBV Express footswitch responds to maybe one in 40 or 50 presses, but of course sets itself to the slowest possible tempo. Front panel TAP button and the app's tap tempo button work fine.
Tuner display on the FBV also behaves strangely. The E, A, & D strings read correctly, but the B string displays 'P', and G string displays '6'. If the tap tempo light is enabled in the app, the display also shows each string to be flatted, though they're tuning correctly to A=440. If the Tap Tempo light is disabled, the flat light works correctly.
Same behavior with 2 different cat5 cables (both 2 meter).
I'd been running 2.50.2 firmware on the TT. I just updated to 2.61.0 and will test again this evening to see if the update helps.
Lastly, and I'm not kidding, my dog chewed through the cat5 cable between the FBV and the TT units. The unit would have been powered off when this happened, and I don't think I powered it up while the cat5 was connected, but if I did, I suspect this could have caused a short and fried something in the TT's FBV pedal port (probably not in the pedal itself as the cable was cut totally in half; no voltage could have gotten to the pedal from the TT).
I've unfortunately got no other L6 gear to test the pedal with to see if the issue is pedal or TT.