Check Bengt Wärleby's post in this topic: Multiplexing Error: Apple Support Communities.
Follow this workflow to help ensure the best quality video DVD:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image and launch DVD Player and play it. If it plays OK with DVD Player the encoding was good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
Also I'll ask the Hosts to move this topic to the iDVD community where it belongs.
I did as you sugested in your first paragraph and still got the same error. "Error during rendering/encoding of the menus/slideshows. Burning process has been canceled." Do you have any other suggestions?
Definitely do not use Memorex or RW disks. Check Bengt Wärleby's post to see if there's anything in your project that might be the culprit. Where did the movie file come from. You need to check out that post.