There is a slight bug in having FTP remote folders bookmarked.
For example, I create a remote FTP location, login to it, do some stuff and close pcmanfm. After short time, the FTP server will close connection because of inactivity. Then, after I open pcmanfm again for example an hour later and select the FTP folder, the message will say that FTP has closed the connection, forcing the user to select something else and then select the FTP folder again.
The solution would be obviously to disconnect from the FTP server upon leaving the remote directory.