System cannot boot: Failed to load SELinux policy, freezing

Issue

• After enabling SELinux, the system cannot boot any longer. These messages appear on the screen:

[  OK  ] Reached target Switch Root.
         Starting Switch Root...
[!!!!!!] Failed to load SELinux policy, freezing

and the booting process does not continue.
* Userspace SELinux utilities report errors like

<module>:ValueError: Failed to read //etc/selinux/targeted/policy/policy.31 policy file