5e4fc9a created the issue

Select a file from the main Dir-Contents panel, then Menu item [File-Duplicate], then select [Duplicate] from the filename dialog.

Results
Error Dialog: "FTP Error: Cannot copy ".

If you then select "Cancel" or "Try Again", the Dir-Contents panel is cleared out. Upon reconnecting: the directory contents are displayed correctly, and the file has not been duplicated.

I have performed this test on three different FTP servers and the results are the same. So far, this is 100% repeatable.