Just for fun I tried upgrading software-properties-gtk to version 0.96.24.8 (along with both dependencies) from "http://us.archive.ubuntu.com/ubuntu/pool/main/s/software-properties/". Unfortunately that didn't make any apparent difference.
I also tried downgrading to version 0.96.20.5, which also exhibits the same failure.
Just for fun I tried upgrading software- properties- gtk to version 0.96.24.8 (along with both dependencies) from "http:// us.archive. ubuntu. com/ubuntu/ pool/main/ s/software- properties/". Unfortunately that didn't make any apparent difference.
I also tried downgrading to version 0.96.20.5, which also exhibits the same failure.