/dev/null: Permission denied

The problem seems to be with the permissions of
the /dev/null. This seems to be read only at the
moment for you. Check this by logging in as root
and listing it with the command:
ls -l /dev/null
You should see this if everything is correctly
set:
crw-rw-rw- 1 root root 1, 3

If you get a different set of permissions like
this maybe:
-rw-r--r-- 1 root root 1, 3

then you should (as root) delete the /dev/null with:
rm /dev/null

and recreate it (as root) again with:
mknod -m 0666 /dev/null c 1 3

(The device number according to the Kernel source
in the documentation under Documentation/devices.txt
supposed to be Major=1 und Minor=3)

Now, list the /dev/null again and you should see
the permissions as above. Hope this helps..