14.04.2 over 15.04 installation, grub signed image install crashed, dual boot windows 8.1

[Luca]

14.04.2 over 15.04 installation, grub signed image install crashed, dual boot windows 8.1

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity 2.18.8.6 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 3.16.0-30.40~14.04.1-generic 3.16.7-ckt3
Uname: Linux 3.16.0-30-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.7
Architecture: amd64
CasperVersion: 1.340
Date: Mon Jul 20 16:25:35 2015
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash --
LiveMediaBuild: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
ProcEnviron:
 LANGUAGE=it_IT.UTF-8
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=it_IT.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)