I upgraded Ubuntu 11.10 to 12.04 LTS immediately after its release (april 2012). Since then updates (new kernels and others) succeeded regularly, but recently, suddenly, updates fail consistently.

What causes the problem and how can it be solved?

Terminal information after the last update attempt:

ariel@ariel-MS-7592:~$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
3 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue [Y/n]? y
Setting up linux-image-3.2.0-26-generic (3.2.0-26.41) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/dkms 3.2.0-26-generic /boot/vmlinuz-3.2.0-26-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 3.2.0-26-generic /boot/vmlinuz-3.2.0-26-generic
update-initramfs: Generating /boot/initrd.img-3.2.0-26-generic
run-parts: executing /etc/kernel/postinst.d/pm-utils 3.2.0-26-generic /boot/vmlinuz-3.2.0-26-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 3.2.0-26-generic /boot/vmlinuz-3.2.0-26-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 3.2.0-26-generic /boot/vmlinuz-3.2.0-26-generic
/usr/sbin/grub-mkconfig: 11: /etc/default/grub: splash”: not found
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 127
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-3.2.0-26-generic.postinst line 1010.
dpkg: error processing linux-image-3.2.0-26-generic (--configure):
 subprocess installed post-installation script returned error exit status 2
No apport report written because MaxReports is reached already
                                                              dpkg: dependency problems prevent configuration of linux-image-generic:
 linux-image-generic depends on linux-image-3.2.0-26-generic; however:
  Package linux-image-3.2.0-26-generic is not configured yet.
dpkg: error processing linux-image-generic (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of linux-generic:
 linux-generic depends on linux-image-generic (= 3.2.0.26.28); however:
  Package linux-image-generic is not configured yet.
dpkg: error processing linux-generic (--configure):
 dependency problems - leaving unconfiguredNo apport report written because MaxReports is reached already
                         No apport report written because MaxReports is reached already

Errors were encountered while processing:
 linux-image-3.2.0-26-generic
 linux-image-generic
 linux-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)