Comment 13 for bug 1418706

So from what I can see, the underlying error message from partman-efi happens any time there is NOT an existing EFI system partition on the disk you are installing to, and ubiquity is broken because it displays the error message and waits a few seconds, then proceeds to the next screen, and if you didn't answer the message in that time, after it has moved to the next screen, the window displaying the message becomes broken and you no longer can click either to proceed or go back. partman-efi seems to be displaying the message because it asks parted to format an ext2 partition on the disk, which will later be formatted as fat32 and become an efi system partition, but at the time that partman-efi/init.d/efi runs, parted still reports the partition as ext2, so the script counts it as an existing non EFI system partition, and throws the warning.

So the bug appears to be in two places: both partman-efi should not be throwing the warning, and ubiquity fscks up when the warning is thrown.