I was playing around with internal/external spdif clock to get my Kemper to work with the Studio 68c, I did it from Universal Control. First I couldn't do that but after downgrading the firmware to 2.2.1 from 2.2.2? I could, it worked but the day after the unit was gone under Sound and I couldn't chose or use it.
Tried everything but no luck, bought a new USB cable but still nothing.
Then I found under Applications -> Utilities - -> Audio/midi - setup, there I could change to Presonus internal clock instead of Presonus S/PDIF clock and then I could choose the Presonus as audio unit again.
I google like crazy on this, so I wanted to write this if someone has the same problem. After two days I can make music again :)
PS. I'm from Sweden so sorry if my English sucks....