Tvheadend

A quick fix would be to remove Tvheadend and reinstall after the upgrade, as long as you don't --purge during removal your configuration should remain intact but backup anyway.

4.1.18 is old btw!

I removed both tvheadend & oscam and deleted their entries in /etc/apt/source.list & /etc/apt/source.list.d but still get the annoying error:

Calculating the changes
No candidate ver: libapt-inst1.5
No candidate ver: libapt-pkg4.12
No candidate ver: libboost-iostreams1.55.0
No candidate ver: libcwidget3
No candidate ver: libhogweed2
No candidate ver: libicu52
No candidate ver: libllvm3.6
No candidate ver: libnettle4
No candidate ver: libsigc++-2.0-0c2a
No candidate ver: libxapian22
No candidate ver: oscam-svn
No candidate ver: tvheadend

What is wrong ?

Looks like tvheadend and oscam are still in your system. Check with dpkg if they are really gone:

dpkg -l | grep "tvheadend"
dpkg -l | grep "oscam-svn"

rajan@tv-server:~$ dpkg -l | grep "tvheadend"
rajan@tv-server:~$ dpkg -l | grep "oscam-svn"
rajan@tv-server:~$