Update: Daniel, I ran into this error again. In this case, the file path was:
C:\Users\Generic User\Dropbox (Generic User)\TEXT\Dorico lead sheets - Name\Come, We That Love the Lord - chord demo.dorico.
Other files in this same folder that did not have a comma in the filename worked fine. When I removed the comma in the filename, I was able to open the offending file successfully.
Not sure if that’s helpful…