Sample URL
http://localhost/ckfinder/core/connector/aspx/connector.aspx?command=Thumbnail&type=Images¤tFolder=%2F&langCode=en&hash=2d65929a56e06f0a&FileName=Annual%20Dinner%20Invita#881803 (3)(1).jpg
which gives the following server error
Server Error
404 - File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.
If the # is encoded to %23, it does work