Identical (perhaps) to the bug in the D6 version, which was considered "closed" although I don't see any actual patch there to fix it.
What's really weird is that the view was working fine a month or two ago, but tonight I noticed that it was generating PDFs without the background graphics I had designated before. So I went into the Views UI and found that my background graphic files were no longer listed under settings. I thought somehow the files must've been deleted. So I re-uploaded them and saved the view... and now I'm getting the error message in title.
UPDATE: I updated from rc1 to 7.x-1.0 and the problem persists.