space_cowboy, did you move your Stylus RMX User Libraries to a new location/drive recently?  You mentioned that you use external files, if they are on an external hard drive, it's possible windows assigned a different drive letter when plugging it in.

space_cowboy, I use savihost (which is a vst wrapper) to run Stylus RMS without using a DAW, a kind of standalone mode.  This makes it easier to test when problems arise.  I also use it for creating beats without having to run Sonar.  You may want to give it a try, if Stylus RMX gives the same error when using savihost, you will at least know the problem isn't with Sonar.   

Google says that error message is associated with having shortcuts and/or junctions that point to non-existent files. 
 
You most likely have a shortcut that points to a missing file/nowhere on your hard drive.  It could be anywhere though.  Most likely it's somewhere Stylus RMX (which I don't have) is searching for files.  Search for *.lnk files where ever Stylus RMX is looking for stuff and see if you can find one that is wrong.  The caption of the error message might help you figure how who exactly is showing the message (is it Stylus or is it SONAR?).
 
If you still can't find the problem, then I'd recommend running a tool like Microsoft's Process Monitor, which can record/log every file access a program makes.  You should be able to see where it's looking just before the error comes up.  But ProcMon is very complicated and hard to use so I don't recommend it for most users.
 
https://docs.microsoft.co...nals/downloads/procmon

Solved- though why this happened is a mystery to me.
 
Swapping emails with Spectrasonics Tech Support last night.  They directed me to a Program Data directory with Spectrasonics info to delete some *.dat files.  
 
I noticed that my shortcut to the SAGE convertor looked different than the one to the STEAM folder.  For some reason, the shortcut to sage was empty.  When I created a new shortcut, everything turned copacetic immediately.
 
So, my unresolved question is "Why did the shortcut go bad?"
 
I did not even know there was a program data directory for Spectrasonics.  I could not have intentionally deleted the shortcut.
 
There was no info in the shortcut.  Clicking it went to nowhere.  Whereas clicking the STEAM shortcut sent me to my sample drive where the STEAM folder lives.
 
I had not moved my SAGE files (also on my sample drive), so the short-cut could not have gone bad because the location changed.
 
I had been using RMX a few days before it stopped working and started generating teh "Windows Can't Find..." error, so up until the problem sprung up, I had a working installation.
 
I am old and senile (what were we talking about?  Oh yeah), and I make my share (or more) of mistakes, but I do not see how I could have caused this one.
 
In the few days before  this problem occurred I had installed the early release of SPLAT for July.  And during the period surrounding this issue, my computer automatically updated Win 10.  
 
Something changed that shortcut from a valid shortcut to an invalid and empty file.  
 
You PC experts - any ideas?  I want to avoid things like this again.  I bet I spent 10-15 hours messing with the RMX problem.  
 
Thank