"404 Not Found" when doing "apt-get update"

[Volker Wysk]

Hello!

I'm tyring to do a "apt-get update", and it mostly works. However, there is 
one error message, which I don't understand:

---------------------------
/etc/apt % LANG=C apt-get update
...
Ign:19 http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu xenial/main DEP-11 
64x64 Icons
Err:12 http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu xenial/main amd64 
Packages
  404  Not Found
Ign:13 http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu xenial/main i386 
Packages
Ign:14 http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu xenial/main all 
Packages
...
Fetched 1537 kB in 2s (688 kB/s)
Reading package lists... Done
W: The repository 'http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu xenial 
Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore 
potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration 
details.
W: http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/
Ubuntu_16.04/Release.gpg: Signature by key 
F9EA4996747310AE79474F44977C43A8BA684223 uses weak digest algorithm (SHA1)
E: Failed to fetch http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu/dists/
xenial/main/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones 
used instead.
---------------------------

What's going wrong here? Do I have an incorrect line in my /etc/apt/
sources.list?

Also, I assume that "Ign:" means "ignored". Is this okay?

May thanks,
Volker