I'm also experiencing this issue. It's down to there being a '&' in the podpress media title/mp3. I know this because the offending line in the error message relates to a file with a '&' in. If I go back into the post, podpress has automatically changed the '&' in the title (and mp3 route) to '& amp;' (without the space). If I simple 'Update' the post, this fixes the issue. Weirdly this doesn't always happen. I have several MP3s with '&' in and it happens on quite a lot of them, but not all of them.
Summary: Look at the line in the feed that the error is referring to and work out what post it is coming from. Open that post in question. Click 'update'. Refresh your feed. Problem solved.
Obviously it would be better if the plugin performed correctly and you didn't have to do this!