problems upgrading from 14.04 to 16.04

[J.L. Blom]

I finally bit the bullet and upgraded from 14.04 to ubuntu 16.04. As I'm 
using Xubuntu I assume the upgrade is to Xubuntu. Helas, that was met 
with a lot of errors.
It didn't use the graphic mode when it started and apt-get didn't work. 
With help of the internet I solved bot.
However, I remain stuck with the following error/problem:
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joep at laguna:~$ sudo apt -f install
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... failed.
The following packages have unmet dependencies:
  libstdc++6 : Depends: gcc-5-base (= 5.4.0-6ubuntu1~16.04.2) but 
5.4.1-2ubuntu1~14.04 is installed
               Breaks: libkolabxml1 (<= 1.1.0-3) but 1.0.1-0ubuntu3 is 
installed
               Breaks: libstdc++6:i386 (!= 5.4.0-6ubuntu1~16.04.2) but 
6.2.0-3ubuntu11~14.04 is installed
  libstdc++6:i386 : Breaks: libstdc++6 (!= 6.2.0-3ubuntu11~14.04) but 
5.4.0-6ubuntu1~16.04.2 is installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be 
caused by held packages.
E: Unable to correct dependencies
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When I try to solve the "held packages" sentence I get:
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joep at laguna:~$ sudo apt update
[sudo] password for joep:
Err:1 http://nl.archive.ubuntu.com/ubuntu xenial InRelease
   Could not resolve 'nl.archive.ubuntu.com'
Err:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
   Could not resolve 'security.ubuntu.com'
Err:3 http://archive.canonical.com/ubuntu xenial InRelease
   Could not resolve 'archive.canonical.com'
Err:4 http://dl.google.com/linux/chrome/deb stable InRelease
   Could not resolve 'dl.google.com'
Err:5 http://nl.archive.ubuntu.com/ubuntu xenial-updates InRelease
   Could not resolve 'nl.archive.ubuntu.com'
Err:6 http://dl.google.com/linux/earth/deb stable InRelease
   Could not resolve 'dl.google.com'
Err:7 http://nl.archive.ubuntu.com/ubuntu xenial-backports InReleasechrome
   Could not resolve 'nl.archive.ubuntu.com'
Reading package lists... Done
Building dependency tree
Reading state information... Done
52 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: Failed to fetch 
http://nl.archive.ubuntu.com/ubuntu/dists/xenial/InRelease  Could not 
resolve 'nl.archive.ubuntu.com'
W: Failed to fetch 
http://nl.archive.ubuntu.com/ubuntu/dists/xenial-updates/InRelease Could 
not resolve 'nl.archive.ubuntu.com'
W: Failed to fetch 
http://security.ubuntu.com/ubuntu/dists/xenial-security/InRelease Could 
not resolve 'security.ubuntu.com'
W: Failed to fetch 
http://nl.archive.ubuntu.com/ubuntu/dists/xenial-backports/InRelease 
Could not resolve 'nl.archive.ubuntu.com'
W: Failed to fetch 
http://dl.google.com/linux/chrome/deb/dists/stable/InRelease  Could not 
resolve 'dl.google.com'
W: Failed to fetch 
http://dl.google.com/linux/earth/deb/dists/stable/InRelease  Could not 
resolve 'dl.google.com'
W: Failed to fetch 
http://archive.canonical.com/ubuntu/dists/xenial/InRelease  Could not 
resolve 'archive.canonical.com'
W: Some index files failed to download. They have been ignored, or old 
ones used instead.
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However, The URL 
http://nl.archive.ubuntu.com/ubuntu/dists/xenial/InRelease does exist so 
the reason why it couldn't be resolved is not clear. The same goes for 
security.ubuntu.... and archive.canonical.com....
The google sites (dl.google.com/linux...) were not reachable (all using 
firefox as browser).
So I don't know where the problems are but at least the entries in the 
settings are not in order and I have no clue how to solve this problem.
I hope somebody in the group can help. Of course I followed the standard 
lines to report to the developers but I don't think I'll get much help 
from there.
Joep