Just started getting this issue the past 2 days only with Coffeecup - other FTP programs working fine (Filezilla, internal ftp in pspad and phpdesigner as well functioning)...

The PORT command returned an error on your server. This could mean the connection has been dropped or is hung up on the host's end. Disconnect and try reconnecting. [30] Additional information: Failed to do PORT.
Server has replied: [500] I won't open a connection to 192.168.0.137 (only to xx.xx.xx.xx / this number is the external ip of the modem).

Anyone else seeing this out of the blue?

Not me John, just loaded up a few different domains with mine and nothing here, sounds like it might be an issue on your server maybe, especially if it was working fine until the last couple days. Might want to contact them and see what they have to say about it, in the meantime someone else here might know what that message means as to what it pertains to being the software or the server. Good luck on it.