Most probably a corrupted pref file.
Share to iDVD - is no good way to go - the DVD will not be as good as it can be from iMovie'11 (9.0.9)
Do: Share to Media Browser - and as MEDIUM - HD will not improve either as iDVD does a bad job in Scaling Down.
Then in iDVD - You find Your movie under the Media button (down to the right) and here Movies.
Use a name that is easy to remember - You can change it when in iDVD.
Yours Bengt W