The problem is actually not with mixxx itself, but with libtag1. Mixxx supplies a version of libtag1 as a prerequisite. But in Ubuntu there is already libtag1v5, which is a newer version. Apt is refusing to downgrade to libtag1.

So the solution must be for mixxx to either use the installed libtag1 or rename the libtag1 it needs to something else to avoid naming conflicts.


==== Install attempt from ppa:

[root] # cat /etc/apt/sources.list.d/mixxx-ubuntu-mixxx-xenial.list
deb http://ppa.launchpad.net/mixxx/mixxx/ubuntu xenial main
# deb-src http://ppa.launchpad.net/mixxx/mixxx/ubuntu xenial main

[root] # apt install mixxx
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 mixxx : Depends: libtag1-vanilla (>= 1.10-0ubuntu1~xenial0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

==== Install attempt from downloaded deb package:

[root] # dpkg -i /home/markus/Downloads/mixxx_2.1.1-0ubuntu1_2.1_git6743_xenial_amd64.deb 
Selecting previously unselected package mixxx.
(Reading database ... 520479 files and directories currently installed.)
Preparing to unpack .../mixxx_2.1.1-0ubuntu1_2.1_git6743_xenial_amd64.deb ...
Unpacking mixxx (2.1.1-0ubuntu1~2.1~git6743~xenial) ...
dpkg: dependency problems prevent configuration of mixxx:
 mixxx depends on libtag1-vanilla (>= 1.10-0ubuntu1~xenial0); however:
  Package libtag1-vanilla is not installed.

dpkg: error processing package mixxx (--install):
 dependency problems - leaving unconfigured
Processing triggers for menu (2.1.47ubuntu1) ...
Processing triggers for desktop-file-utils (0.22-1ubuntu5.2) ...
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20180209-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for gnome-menus (3.13.3-6ubuntu3.1) ...
Processing triggers for mime-support (3.59ubuntu1) ...
Errors were encountered while processing:
 mixxx

==== Install attempt of libtag1-vanilla

root@W530:~# apt-get install libtag1-vanilla
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
 libtag1v5-vanilla : Breaks: libtag1-vanilla but 1.10-0ubuntu1~xenial0 is to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

==== Details of libtag1 packages

[root] # apt depends libtag1v5-vanilla
libtag1v5-vanilla
  Depends: libc6 (>= 2.14)
  Depends: libgcc1 (>= 1:4.1.1)
  Depends: libstdc++6 (>= 5.2)
  Depends: zlib1g (>= 1:1.1.4)
  Conflicts: <libtag-extras0>
  Conflicts: <libtag1-rusxmms>
  Breaks: libtag1-vanilla
  Breaks: libtag1c2a (<< 1.5-5)
  Replaces: libtag1-vanilla
  Replaces: libtag1c2a (<< 1.5-5)
[root] # apt depends libtag1-vanilla
libtag1-vanilla
  Depends: libc6 (>= 2.14)
  Depends: libgcc1 (>= 1:3.0)
  Depends: libstdc++6 (>= 5.2)
  Depends: zlib1g (>= 1:1.1.4)
  Conflicts: <libtag-extras0>
  Conflicts: <libtag1-rusxmms>
  Breaks: libtag1c2a (<< 1.5-5)
  Replaces: libtag1c2a (<< 1.5-5)