Currently Being ModeratedAug 10, 2013 7:44 PM (in response to Nemo108)
Xserve EFI firmware doesn't have a CSM that other Mac models have. A CSM presents a BIOS to OS's that require it. So you need to figure out how to do an EFI only installation of Ubuntu. A big gotcha is that I'm betting this Xserve has 32-bit EFI, and most linux distributions only support 64-bit architectures including the EFI. There is a way to compile an EFI supporting bootloader in 32-bit, and then have the rest of the OS in either 32-bit or 64-bit architectures. But it's unusual to find 32-bit EFI bootloaders precompiled.
I'd also suggest newer kernels as they should have better support, but this is rare hardware so decent chance of bugs that no one has encountered. I'd definitely look through dmesg once you get it booting, and make sure there are no major errors (warnings are usually benign).