When crontab gives fopen: permission denied error

While trying to open my user's crontab on Ubuntu, the system barked back:

$ crontab -l
crontabs/myuser/: fopen: Permission denied

This is due to improper permissions on crontab executable.

To fix:

$ sudo chown root:crontab /usr/bin/crontab
$ sudo chmod 2755 /usr/bin/crontab

Mucked up perms:

rwxr-xr-x 1 root root 39352 Nov 16  2017 /usr/bin/crontab

After fix:

-rwxr-sr-x 1 root crontab 39352 Nov 16  2017 /usr/bin/crontab

Solution from Ask Ubuntu: cannot access crontab after changing uid.