UM 16.04 on Lenovo Thinkpad X61: systemd error message and boot problems

I installed UM 16.04 on an older Lenovo Thinkpad X61 with a new Samsung EVO 850 SSD. ‘Trying UM without installing’ worked out of the box. The installation was completed and everything seemed fine, but after I removed the installation USB, the following error appears:
/sbin/casper-stop: 33: /sbin/casper-stop: file: Input/output error
/sbin/casper-stop: 39: /sbin/casper-stop: file: cannot create /lib/systemd/systemd-reply-password: Input/output error
(...) systemd-resolved: Input/output error
(...) systemd-rfkill: Input/output error
(...) systemd-shutdown: Input/output error
(...) systemd-sleep: Input/output error
(...) system-socket-proxyd: Input/output error
(...) systemd-sysv-install: Input/output error
(...) systemd-timedated: Input/output error
(...) systemd-timesyncd Input/output error
and lots more:
systemd-journald 851: Failed to send WATCHDOG=1 notification message: Transport endpoint is not connected.

Is there any way to reboot the system through a key combination? What seems to be the problem?

Update: I don’t know what I did but for some reason it booted after many attempts and UM 16.04 is now stable on this laptop after fixing broken packages through the Software Boutique and manually updating the Welcome package. Does anyone know what could have caused this problem? I have a very vague idea. Could the boot problem be related to an empty BIOS battery? Does anyone understand the error messages I posted earlier? What is systemd?