I cannot identify the reason for this, as I didn't change anything special.
On booting, Ubuntu is stuck in the loading screen. With F1 I get
starting startpar bridge for notification of upstart job start/stop stopping startpar bridge for notification of upstart job start/stop
When I switch to command line I can log in, but I have no idea how to fix this issue there. Network is not working then either, although the computer is connected via cable. Maybe the network config is not loaded (DHCP is off). So I cannot run updates or reinstall.
I tried all options in recovery mode. Enable networking is not working.
I can think of something like this: 1. enable networking 2. reinstall boot loader stuff (GRUB, upload, ...)