I receive this kind of error when I leave the ftp connection alone and the server terminates the connection.
Linux server running ftp client vsftpd. Timeout set to 15min. This is managed by the service provider, they don't want to change this setting.
When I leave NP++ open more than 15 min - or worse - the file editing takes more than 15 min -, the upload or download attempt fails and NP++ becomes freezed. Note that this also happens when I try to open a dir in the folder structure
This annoying proplem can be solved with a background send-receive of some ftp commands just to keep alive the connection.
Currently I solve the problem by disconnecting-reconnecting, but this is also annoying since the opened dir. structure resets to a completely collapsed state...