DVD dual layer with iDVD 6

Hi everyone,

I tried to burn a DV-movie on a dual layer DVD (DVD-R DL) using iDVD 6.0.4 (797).
When I first inserted the DVD and looked into the System Profiler it said that the following:

MATSHITA DVD-R UJ-857E:

Firmware-Version: ZA0E
Verbindungstyp: ATAPI
Brennen möglich: Ja (Laufwerk von Apple geliefert)
Cache: 2048 KB
DVD lesen: Ja
Beschreibbare CD: -R, -RW
Beschreibbare DVD: -R, -R DL, -RW, +R, +R DL, +RW
Schreibstrategien: CD-TAO, CD-SAO, DVD-DAO
Medien:
Typ: DVD-R DL
ID: MKM 01RD30
Leer: Ja
Kann gelöscht werden: Nein
Kann überschrieben werden: Ja
Kann angefügt werden: Ja
Brenngeschwindigkeiten: 2x, 4x

Hence, the system accepted the DVD correctly as a dual layer disc. Then I turned to iDVD, composed my DVD with the following settings:
PAL video, 16:9, Coding: Best performance, DVD type: dual layer. Furthermore the project info told me that the duration was 112:32 minutes, 6,2 GB of 7,7 GB would be used, 1:00 of 7:30 animated menus would be used, 1 of 99 tracks and 2 of 99 menus would be used.

So far the exact setting. Then I tried to burn the DVD. To my surprise the following message occurred (translated): "Your project is too large for a single layer disk. Please insert a dual layer disk!"

What went wrong?? Apparently the system recognizes the disk correctly (according to the System Profiler). How can it be that iDVD does not recognize the disk correctly?!?
Does anyone have any idea on how to solve that issue? I would appreciate this very much!

Thanks for your ideas!

Best regards,
Matthias