You're using VirtualBox (e.g. between a Linux host et a Windows guest) and from times to times, copy/paste stops working ! This has been an ongoing problem that's obviously not resolved: here's a workaround.
You only need to kill the
VBoxTray.exe process (using the Windows Task Manager), and then restart it.
Get the details (if needed) from a WinTips post.
Happy copy/paste !