Is if the filename or the name of one of the folders that contains the comma? Have you by chance tried this in V4 to see if that same file imports correctly there?
I have the same issue with V5 - unable to drag and drop .stl files if the windows file path includes a comma. S3D error "Error opening model with unknown file type..."
Also can't double-click a factory file and have it open. S3D throws a popup "Error opening project with unknown file type..." The error includes the windows file path up to, but not including the comma or anything after it.
V4 has no issues with either of these basic functions.
I just imported a STL and factory file, both which contained a comma in the filename and it worked okay. Can you confirm what OS you are using? Are you using the latest 5.1.2 version?