Well that's interesting. Rebooting the phone made the app willing to save the parameters. Weird.

Oops forgot, this is version 2.5.

I'm having the same problem. I know for fact that the IP address is correct, because I can connect to my computer with an SSH client from the phone. AndFTP is willing to store a server name but not the plain IP address, which is weird. I can't use the server name because this is my home network and ...