I am unable to open the CAD file using drag-and-drop.

Try running the application with administrator rights.

• file access:   Try top open your file with any other application, e.g. a hex.viewer or even notepad 
  • can you open it?   What is displayed : screen snot required
• file read access: 
  • copy n’ paste the following snippets into your Modeling command line:
    using YOUR file, and use FORWARD slashes in the full file path name
    • (display (oli::sd-inq-file-status "c:/work/yourfile.xyz" :existence))    => should be T
    • (display (oli::sd-inq-file-status "c:/work/yourfile.xyz" :read-access))    => should be T
  • what are the results?
• file type:  what kind of files do you try to open?
  • package files ‘*.pkg’ ?
  • bundle files ‘*.bdl’ ?
  • SD files ‘*.sd*’ ?
  • MI files ‘*.mi’ ?
  • BTW : ‘CAD’ file is not the right answer!
• try to open the files via the User Interface => “File”  => “Open”
  • what kind of “File type do you choose” ?
  • send a screen shot of your situation JUST before you press “Load”

Last but not least:

• provide the EXACT error message (as text or as screen shot)  -- the information you gave is not sufficient
• did drag n’ drop of the (same) files work 1 week ago / 1 month ago?
• which system / Modeling version has written the file?