Siduction/Debian has a rescue tool that is not very well known. It comes with systemd and is called debug-shell.service. It allows you to open a rescue shell on tty 9 if you have problems with booting your system. The service file is preinstalled, but not activated. So to be able to use it in case you need it, you should activate it now:
systemctl enable debug-shell.service
The system will set a symbolic link:
ln -s '/lib/systemd/system/debug-shell.service' '/etc/systemd/system/sysinit.target.wants/debug-shell.service'
Now you can at any time open a shell with CTL ALT 9 and try to debug what is wrong by checking the status of services, reading logs, looking for stuck jobs with systemctl list-jobs etc. As long as GRUB starts, you can also add debug-shell.service
to the kernel boot line.
greetz
devil