nvidia-346 346.59-0ubuntu1: nvidia-346 kernel module failed to build {Fails at: "Usage: mountnfs-bootclean.sh [start|stop]"}

That's correct

Status changed to 'Confirmed' because the bug affects multiple users.

Still the same, when I run dist-upgrade this is the output: (I'm also attaching the make.log)

nico@DarkTower:~$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 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-3.19.0-28-generic (3.19.0-28.30) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
Not updating initrd symbolic links since we are being updated/reinstalled
(3.19.0-28.30 was configured last, according to dpkg)
Not updating image symbolic links since we are being updated/reinstalled
(3.19.0-28.30 was configured last, according to dpkg)
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
run-parts: executing /etc/kernel/postinst.d/dkms 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
Error! Bad return status for module build on kernel: 3.19.0-28-generic (x86_64)
Consult /var/lib/dkms/nvidia-346/346.59/build/make.log for more information.
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
update-initramfs: Generating /boot/initrd.img-3.19.0-28-generic
run-parts: executing /etc/kernel/postinst.d/pm-utils 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
run-parts: executing /etc/kernel/postinst.d/unattended-upgrades 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 3.19.0-28-generic /boot/vmlinuz-3.19.0-28-generic
Generating grub configuration file ...
sed: read error on /lib/plymouth/themes/default.grub: Is a directory
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 4
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-3.19.0-28-generic.postinst line 1025.
dpkg: error processing package linux-image-3.19.0-28-generic (--configure):
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 linux-image-3.19.0-28-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)