I'm having problems with DVD movie playback. Apparently my system mounts the DVD, as the icon pops up on the desktop, but when I try to watch the movie it says "an error occoured, cannot read from resource". I can browse the the DVD files, so this is most likely a problem with totem. I have no clue on why, or how to begin troubleshooting. =/ Help?

My system is a Gateway c-141 XL convertible running Ubuntu 8.04 with a 8x Multi-Format Dual Layer DVD-RW with DVD-RAM. Don't really know if this is relevant.