After rebooting, the "copy and paste" functionality began working again.
However, I want to emphasize that this remains an outstanding bug in the main repositories of Ubuntu 15.10, 15.04, 14.10, and I think even 14.04.3.
Since the Remmina Next repository works better than what is in the main repositories, I don't understand why "someone with the authority to do so" doesn't just replace what's in the main repositories with what's currently in Remmina Next.
After upgrading to Ubuntu 15.10, I enjoyed "copy and paste" working for about 2 or 3 weeks, and then it stopped again.
To fix I had to upgrade to the Remmina Next repository, which I installed like this:
sudo apt-add-repository ppa:remmina- ppa-team/ remmina- next ; sudo apt-get update ; sudo apt-get install remmina remmina-plugin-rdp
After rebooting, the "copy and paste" functionality began working again.
However, I want to emphasize that this remains an outstanding bug in the main repositories of Ubuntu 15.10, 15.04, 14.10, and I think even 14.04.3.
Since the Remmina Next repository works better than what is in the main repositories, I don't understand why "someone with the authority to do so" doesn't just replace what's in the main repositories with what's currently in Remmina Next.