I am using a home wireless network connected to a cable Internet connection. This is what I normally use most of the time I am using WordPress, including "Copy A Post" The last time I used the Copy function was 2-3 weeks ago.
No difference whether or not I am using http or https.
I am using Firefox. somewhat out of date, but the problem also occurs with an up to date Internet Explorer. Again, these are the browsers I normally use I will update Firefox, but am trying to isolate any smaller issues in your list before changing anything else.
Cache and cookies cleared to no effect.
Third party cookies and Java enabled, and have been since before this problem began. Flash is up to date.
No recent change in operating system, (Windows 7) unless Windows has autoupdated something of which I'm not aware Security systems up to date--Windows Firewall, AVG Free, autoupdated earlier today. I have not done any manual installs or uninstalls of any software recently.