What you describe suggests that a poorly designed add-in is interfering with copy and paste operations. As a troubleshooting step, start Word in Safe mode and see if that fixes the problem for the session run in Safe mode.
To start in Safe mode: Exit Word if it is currently running. Hold the Windows logo key as you press R; in the Run dialog box, type
winword /safe and press Enter (note the space before the slash). You'll see "Safe Mode" on the title bar of the Word window.
If Safe mode works, continue troubleshooting COM and Startup add-ins as discussed in the Knowledge Base article at
https://support.microsoft.com/en-us/kb/921541.