I've had issues with multiplexer errors before, and in retrospect, I should have taken better notes.
Since I was using a quicktime reference movie out of FCP, my first step troubleshooting was to try it as a self-contained one instead, which had no effect. Then, thinking it was a problem with my hardware, I moved the whole IDVD project to another computer, but I still hit the same error. This was a couple of years ago, now nearly forgotten at the edge of my own personal memory.
I just re-encountered the error yesterday and had to re-think it through. What ultimately worked (this time) was trashing my render files and rerendering. Then I re-exported, brought it back to IDVD, and the IDVD burn worked like a charm. I'm thinking something was amiss on those render files. I have no other indication that anything was wrong with them other than the Idvd error.
I'm thinking the first time I may have had to go a step further - to put it on a different sequence and rechapter it. Your actual mileage may vary, but I think it has something to do with a glitch in the source files.