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TL;DR: Ubuntu boot hangs on starting the snappy daemon, takes roughly 5min to boot, reinstall doesn't fix, issue is caused by an update.
Backstory: I installed the vanilla-gnome-session and after deciding I didn't like it removed it again. I later had installed and removed ubuntu-gnome-session hoping that it would bring back the old login screen. After that I did an update before reboot. Upon shutdown the loading screen for ubuntu gnome appeared and was running a kernel upgrade. Then when booting again the loading boot screen for ubuntu froze, switching to TTY2 revealed that it hung on three jobs:
A start job is running for * Snappy daemon * Wait until snapd is fully seeded * Hold until boot process finishes up
It takes roughly 5min until this process is finished.
After some unsuccesful troubleshooting decided to reinstall. I first used the reinstall option from the live install disc and after finishing the PC booted fine even after a reboot. However as soon as I ran
apt update && apt upgrade it would hang again.
Some goes after doing a complete reinstall (choosing "erase disk and install" on the install disc)