Version 6.3 on Windows 10 works for me. I just used the all three methods successfully:
- Drag and Drop into Book details panel;
- Add Books->Add files to selected book records from clipboard (that I copied using Ctrl+C in explorer)
- Add Books->Add files to selected book records (that uses the standard file open)
Third party file managers that replace Windows File Explorer have been known to cause these sort of issues.
BR