Internal Server Error: CageFS jail error Unable to determine realpath of /home/USER (No such file or directory)

Symptoms

You will see errors like the following in the Apache error log (/var/log/apache2/error_log).

CageFS jail error Unable to determine realpath of /home/USER (No such file or directory)

Description 

This can be caused by CageFS not being properly mounted.

Workaround

CageFS can be remounted with the following command.

cagefsctl -M