Yup, line is:
Dim fac As New Chilkat.FileAccess
success = fac.WriteEntireTextFile(prefFile.ShellPath,json.Emit(),"utf-8",False)
The output of the ShellPath is exactly /Users/mwilliams/Library/Application\ Support/com.customapp.oauth/ms\-tokens/microsoftGraph.json
FYI, I’ve also tried NativePath and other manual paths. Unfortunately the documentation regarding the root context of WriteEntireTextFile is opaque at best.