I finally figured this out myself.
The easiest straighforward way is:
1. Use the alternate CD (Its not offered for 14.04.2 at this time so use 14.04.1)
2. Select "Install" (from the Unetbootin-menu - Important! - see details)
3. If you're installing from an Image on USB-Pendrive: When you're asked to install grub to MBR select "No" and specify the correct target device.
A little more Detail:
1: The Bug in the Desktop-Installer still exists in 14.04.2 (I tried it). There is a workaround, described here. This should also lead to successfull install.
2: I was Using Unetbootin to install the iso on my usb-pendrive.
It creates it's own bootmenu and offers the options "Install" "Expert Install" and "Install Lubuntu" among others.
"Install Lubuntu" will fail at "select and install software" (this bug
There's another workaround that should lead to successfull install)
So better use "Install" (or "Expert Install"). That will lead to another bug if you use a usb-pendrive for installation but its easy to solve:
3: (heres a bugreport ) Grub will be installed to the MBR of the usb-pendrive (you can boot your PC with the usb-pendrive inserted)
The MBR-bug is easy to solve though by selecting "No" when you're asked if grub should be installed to MBR. This will not cancel the install of grub. You will be asked to specify the target device for grub-install.
Holy Moly - May encryption be with you
I also posted this in ubuntuforums