I should add that on the servers that boot from CD-ROM through the Virtual Media applet from iLO2 I'm able to chroot into the existing installations and everything works as expected. I'm able to reinstall GRUB to the MBR of the first logical drive in every possible way without any error (using the live CD, using the grub-install or grub-setup from the chroot'ed partition etc.). Shooting in the dark I have even tried to install GRUB2 to the MBR of the other logical drives.
On one of these servers I've even deleted all logical drives except the one holding the root partition for the existing Ubuntu Linux installation. I've even forced installing GRUB2 to the boot partition from this single logical drive as seen here: http://h30499.www3.hp.com/t5/ProLiant-Servers-ML-DL-SL/ML350-G3-Attempting-to-boot-from-hard-drive-c-hanging/m-p/5112137#M119241
All in vain, unfortunately... Any help would be much appreciated!