Somehow my Raspberry started giving me this error whenever I tried crontab -e from a user account (sudo crontab -e worked fine for root contab, or sudo crontab -e -u username for offending user).

I searched on the internet several times, trying to fix permissions on crontabs folder, without any joy, until this post finally helped me:

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

and I was able to edit user’s crontab from user’s CLI.