Sometime after host rebooting and always after Virtualbox updating copy/paste from host Ubuntu to guest XP stops to work. After next reboot it usually work as expected. Rebooting guest does not help. Switching "share clipboard" on-off does not help. Rebooting host is usually something to avoid. Can I fix somehow the problem?
I think that the resolution has been not to fix it for XP. A workaround is stopping and restarting
VBoxTray.exe in XP. Working partially; more info here on superuser.sx.
What I used to do was using the Task Manager in Windows XP, killing the
VBoxTray.exe process, and then start it again (with the "run..." option in the Start menu, typing the program name is usually sufficient). Works again during a time, then stops...