All files in the same directory - guess it should work OK, so I have no explanation.
So I am just stabbing in the dark ...
.) Did you let Prime recalculate (F5 or F9) the sheet when you opened "Ref Document.mcdx"? Prime does not do this automatically on start up (like MC 15 did) but rather shows the result before the last saving (which may be the correct result the sheet had in MC15).
.) Another idea is that Prime may have troubles with long path names (should not be the case nowadays with current software, but who knows). As your sheets only use relative paths (with the exception of the call to the referenced sheet) you may try to copy all three involved files to a local directory with a short path name like C:\Test\, adapt the path in the reference call and see if the error persists.
You may also consider permanently edit the path in the reference to be a relative path - according to the docs this should work (only backslashes will be accepted, no forward slashes).
.) It may have to do with Onedrive. I recalled a thread where a user reported about problems he had reading Excel files from Onedrive via READEXCEL. Not sure if it has to do with your problem, but copying the files to a local directory should show.
-> Solved: Read from open Excel file. MP10 - PTC Community
