Same here. I downloaded ca-certificates_20170717~16.04.1_all.deb from https://packages.ubuntu.com/xenial/all/ca-certificates/download
Then did a: dpkg -i ca-certificates_20170717~16.04.1_all.deb dpkg --configure -a
After that I did a: sudo apt-get upgrade
Which upgraded to 20180409 without problems
Same here. I downloaded ca-certificates _20170717~ 16.04.1_ all.deb from https:/ /packages. ubuntu. com/xenial/ all/ca- certificates/ download
Then did a: _20170717~ 16.04.1_ all.deb
dpkg -i ca-certificates
dpkg --configure -a
After that I did a:
sudo apt-get upgrade
Which upgraded to 20180409 without problems