you usually get that message when another piece of software has grabbed on to the file and won't let go of it, you will have to find out what that software is and shut it down.
another possibility is it simply just can't play the file. the sony dvf plugin we are using does not support dvfs from every single recorder out there, why i do not know, but that is how sony chose to operate.
check it is working in media player first, that is usually a sign of whether it is going to work or not.
of course all this is assuming you have already installed the dvf plugin.....