cannot access /dev/null: Permission denied, although crw-rw-rw- 1 root root 1, 3 Sep 21 12:05 /dev/null

The problem came from the permissions to /dev:

# ls -ld /dev
drwx------ 3 root root 4096 Sep 21 12:12 /dev

So /dev was not accessible by users.

# chmod a+x /dev
# chmod a+r /dev

Solved the problem.