When I run software update I get
package operation failed
The updates downloaded ok.
The error window disappeared so fast, I could not write down errors.
I ran sudo apt-get dist-upgrade
Errors were encountered while processing: /var/cache/apt/archives/apt_1.6.6ubuntu0.1_amd64.deb E: Sub-process /usr/bin/dpkg returned an error code (1)
While following the answer below I got:
Unpacking apt (1.6.6ubuntu0.1) over (1.6.6) ... dpkg: error processing archive /var/cache/apt/archives/apt_1.6.6ubuntu0.1_amd64.deb (--unpack): unable to make backup link of './lib/systemd/system/apt-daily.timer' before installing new version: Operation not permitted dpkg-deb: error: paste subprocess was killed by signal (Broken pipe)