crontab listing or editing results in fopen: permission denied

Solution 1:

The permissions on /usr/bin/crontab should be:

 perms= 4755  (rws rx rx)

Your /usr/bin/crontab has the wrong group owner. Additionally, it has sgid and NOT suid which it must have.

Solution 2:

You can edit your user crontab if you have superuser access, without modifying permissions:

$ sudo crontab -u username -e

I thought you could edit your user crontab without sudo, but I get the same permission issues as you, even after adding myself to the crontab group. The permissions in my user's crontab (/var/spool/cron/crontabs) are -rw-------, group is crontab.