This is a known bug which has now been fixed in the forthcoming new release which is due imminently. There is workaround which you can use as shown below:
Please do the following:
Close CompleteFTP Manager if it’s running.
Stop the CompleteFTP service.
Open a console as administrator.
Run the following command: "C:\Program Files (x86)\Complete FTP\Server\cftpconfig.exe" /q "UPDATE [Site] SET [PASVIP]='XYZ.XYZ.XYZ.XYZ' WHERE [System]=0"where XYZ.XYZ.XYZ.XYZ is your external IP address.
Start the CompleteFTP service.
Open CompleteFTP Manager and verify that external IP address has been set.