I have now made a look which files in /etc/initramfs-tools and /usr/share/initramfs-tools belongs to which package. Here is the result:

sworddragon@ubuntu:~$ (find /etc/initramfs-tools -type f; find /usr/share/initramfs-tools -type f) | xargs dpkg -S
initramfs-tools: /etc/initramfs-tools/initramfs.conf
dpkg-query: no path found matching pattern /etc/initramfs-tools/modules
dpkg-query: no path found matching pattern /etc/initramfs-tools/conf.d/resume
initramfs-tools: /etc/initramfs-tools/update-initramfs.conf
cryptsetup: /usr/share/initramfs-tools/conf-hooks.d/cryptsetup
mountall: /usr/share/initramfs-tools/hooks/mountall
kmod: /usr/share/initramfs-tools/hooks/kmod
cryptsetup: /usr/share/initramfs-tools/hooks/cryptkeyctl
cryptsetup: /usr/share/initramfs-tools/hooks/cryptgnupg
cryptsetup: /usr/share/initramfs-tools/hooks/cryptroot
plymouth: /usr/share/initramfs-tools/hooks/plymouth
keyboard-configuration: /usr/share/initramfs-tools/hooks/console_setup
initramfs-tools: /usr/share/initramfs-tools/hooks/framebuffer
cryptsetup: /usr/share/initramfs-tools/hooks/cryptpassdev
udev: /usr/share/initramfs-tools/hooks/udev
cryptsetup: /usr/share/initramfs-tools/hooks/cryptopenct
initramfs-tools: /usr/share/initramfs-tools/hooks/fixrtc
initramfs-tools: /usr/share/initramfs-tools/hooks/thermal
cryptsetup: /usr/share/initramfs-tools/hooks/cryptopensc
initramfs-tools: /usr/share/initramfs-tools/hooks/compcache
kbd: /usr/share/initramfs-tools/hooks/kbd
initramfs-tools: /usr/share/initramfs-tools/hooks/busybox
dmsetup: /usr/share/initramfs-tools/hooks/dmsetup
initramfs-tools: /usr/share/initramfs-tools/hooks/klibc
initramfs-tools: /usr/share/initramfs-tools/hook-functions
mountall: /usr/share/initramfs-tools/event-driven/upstart-jobs/mountall.conf
initramfs-tools: /usr/share/initramfs-tools/modules
initramfs-tools: /usr/share/initramfs-tools/scripts/nfs
initramfs-tools: /usr/share/initramfs-tools/scripts/local
cryptsetup: /usr/share/initramfs-tools/scripts/local-top/cryptroot
cryptsetup: /usr/share/initramfs-tools/scripts/local-top/cryptopensc
cryptsetup: /usr/share/initramfs-tools/scripts/local-bottom/cryptopensc
plymouth: /usr/share/initramfs-tools/scripts/init-bottom/plymouth
udev: /usr/share/initramfs-tools/scripts/init-bottom/udev
initramfs-tools: /usr/share/initramfs-tools/scripts/functions
plymouth: /usr/share/initramfs-tools/scripts/init-top/plymouth
initramfs-tools: /usr/share/initramfs-tools/scripts/init-top/keymap
keyboard-configuration: /usr/share/initramfs-tools/scripts/init-top/console_setup
initramfs-tools: /usr/share/initramfs-tools/scripts/init-top/framebuffer
udev: /usr/share/initramfs-tools/scripts/init-top/udev
initramfs-tools: /usr/share/initramfs-tools/scripts/init-top/blacklist
initramfs-tools: /usr/share/initramfs-tools/scripts/init-top/all_generic_ide
plymouth: /usr/share/initramfs-tools/scripts/panic/plymouth
initramfs-tools: /usr/share/initramfs-tools/scripts/panic/keymap
keyboard-configuration: /usr/share/initramfs-tools/scripts/panic/console_setup
initramfs-tools: /usr/share/initramfs-tools/scripts/local-premount/resume
initramfs-tools: /usr/share/initramfs-tools/scripts/local-premount/fixrtc
initramfs-tools: /usr/share/initramfs-tools/init


I have also reinstalled all these packages with the command "(find /etc/initramfs-tools -type f; find /usr/share/initramfs-tools -type f) | xargs dpkg -S | awk '{print substr($1, 0, length($1))}' | xargs sudo apt-get install --reinstall -y; sudo update-initramfs -u; sudo update-grub" to go sure that the files in /etc/initramfs-tools and /usr/share/initramfs-tools are unmodified. But after rebooting the system and going into the grub menu I'm still seeing the entry "single".