I will try to respond to provide detail in my situation.
I have a rack mount Kemper and Remote pedal.
Attached to the remote, I have 3 expression pedals.
My volume pedal is a Mission GP1-KP. It is plugged into Pedal 3 input.
I use the bottom slots 1- 10ish for the sets we will be playing.
I was using either slot 20 or Slot 2 or 3. (Apologies, I do not recall as I was organizing the songs for this weekend)
Anyway, I upgraded the OS while I was moving profiles around.
Finalizing for the set, I was working on a slide and swells using the volume pedal when the issue occurred.
(Add - after I rebooted, the sound returned)
This was within an hour of the upgrade. Luckily I had my backup version.
I backdated to 8.7.10.38833 and left the system on for several hours, playing intermittently to check sound. So far so good.
I did not notice the input LED's or try to set tempo during the failure. I associated the failure with the OS and immediately backdated as I am booked this weekend. I hope that helps.