grub2 setup to mbr fails with alternate disc on 2. time

[magelan]

Binary package hint: debian-installer

Ubuntu 10.10, Alternate disc

1. Windows 7 only, Windows bootloader.
2. I installed ubuntu 10.10 including luks and lvm and the grub setup worked fine.
3. I had to install it again, now it said something like "Grub setup failed, could not install it to MBR." I made a reboot, executed the alternate setup again and the error reappeared.

So it works when only the Windows 7 bootloader is on the system, but not with grub already on it.

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: debian-installer (not installed)
ProcVersionSignature: Ubuntu 2.6.35-25.44-generic 2.6.35.10
Uname: Linux 2.6.35-25-generic x86_64
Architecture: amd64
Date: Tue Mar 1 21:54:08 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
Lsusb:
 Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: System manufacturer System Product Name
MemoryUsage:
 total used free shared buffers cached
 Mem: 2057128 782744 1274384 0 82540 281996
 -/+ buffers/cache: 418208 1638920
 Swap: 1687544 0 1687544
ProcCmdLine: BOOT_IMAGE=/vmlinuz-2.6.35-25-generic root=/dev/mapper/vg-main ro nosplash
ProcEnviron:
 LANG=de_DE.utf8
 SHELL=/bin/bash
SourcePackage: debian-installer
dmi.bios.date: 12/01/2005
dmi.bios.vendor: Phoenix Technologies, LTD
dmi.bios.version: ASUS A8N-SLI DELUXE ACPI BIOS Revision 1016
dmi.board.name: A8N-SLI DELUXE
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: 1.XX
dmi.chassis.asset.tag: 123456789000
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnPhoenixTechnologies,LTD:bvrASUSA8N-SLIDELUXEACPIBIOSRevision1016:bd12/01/2005:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnA8N-SLIDELUXE:rvr1.XX:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer