It refers to your rom path. I know this because I put the error message in myself.

Here's something that's dumb that might work.... Try to put in two rom paths....
roms=c:\windows\;c:\mame\roms\
It doesn't matter that there isn't anything there this is just for debug pruposes. All that matters is that the folder actually exists.
If it gets through the first vdir fine then we know for some reason it isn't liking the folder you have your roms in. It could be that it's compressed or some other such nonsense.