Och bitte ...
Of course I pulled the cord from the KPA.
No offense intended, I know experienced people that have troubleshoot for 20 min just to realize they did not plug the guitar...
With previous firmware the alt input was disabled unless you first did plug and unplug the front input, now is enabled unless you plug the front input...that means is something wrong with the alt input, can you check if is something inside? if not are you using the same cable front and back? if yes the it must be some contact on the rear plug