I've removed all apt sources.list and added back from current version of Ubuntu which helped me to package by package upgrade it to current Ubuntu version of the package
I had same issues and they were in a system because some packages were from the previous version ( 16 ) which must be upgraded after OS upgrade to 18
so rm -rf /etc/apt/sources.list.d/*
apt-get update
apt-get upgrade
add all repositories back but for current version of Ubuntu and you'll succeed
I've removed all apt sources.list and added back from current version of Ubuntu which helped me to package by package upgrade it to current Ubuntu version of the package
I had same issues and they were in a system because some packages were from the previous version ( 16 ) which must be upgraded after OS upgrade to 18
so rm -rf /etc/apt/ sources. list.d/ *
apt-get update
apt-get upgrade
add all repositories back but for current version of Ubuntu and you'll succeed