This has been happening for the last few releases. I navigate to a folder in S3, select a file, hit space to do Quick Look, and instead of seeing the document or image, I get a file dialog. The dialog doesn't seem to let me actually select any files, so I hit Cancel.
Up until the last week or so, Cyberduck would then show me the file. But now it doesn't even do that; it just pops up an error message. I'll attach screenshots.
If I double-click the file rather than trying to use Quick Look, it downloads successfully to the appropriate folder, so I don't believe this is a problem with the file transfer per se.