I discovered that I'd saved the one song on two separate external hard drives. On one, the 01 series and 16 series were missing. On the other, the 03 series and 16 series were missing. So I overwrote the project file that was missing the 01 series and 16 series with the one that was missing the 03 series and 16 series and now both play... for now.
 
This situation needs to get fixed ASAP.

bump

This wouldn't be a solution - but would it temporarily work for you to remove the checks for a few of the upper channels from your interface - Edit>Preferences>Audio>Devices?
 
I would think this would make it possible for X3 to display the ones that are missing, until somebody works out a real solution for the issue.
.
 
Just a thought.  :)
 
Bob Bone
 

Good idea. If the outputs I need go missing again I'll try that.
The problem seems to be that X3 has to read 3 inputs/outputs for every series.
 
If I go to assign an input channel to record a track, the drop down menu displays Left 01, Right 01, Stereo 01, Left 03, Right 03, Stereo 03, etc.
 
The input drop down menu displays a maximum of 24 items so it only allows 15 out of 16 inputs.
 
I don't have 16 working preamps right now (or a reason to need them) so that isn't a problem right now.

Kewl - you might want to just give it a try, so you know that the technique works (or not).
 
That way, you know ahead of needing it when you are in the middle of something important or urgent.
 
Bob Bone