Thank you for your answer!
When I said "tvheadend" I meant the plugin for kodi. The tvheadend server still works (I have another Ubuntu with Kodi 14.2, and it still works). I use "HTS Tvheadend 3.4.28~geb79aee~trusty" on my backend server.
And by saying "stopped working" I meant my kodi, now version 16.0, gives the error "lost connection" to tvheadend backend.
I tried to downgrade kodi on my Ubuntu, but didn't get it to work. Maybe I should modify the version-number-string in some way?
holm@frontend:~$ sudo apt-cache policy kodi*
N: Unable to locate package kodi_crashlog-20160206_080033.log
N: Couldn't find any package by regex 'kodi_crashlog-20160206_080033.log'
holm@frontend:~$ sudo apt-cache policy kodi
kodi:
Installed: 2:16.0~git20160220.1654-final-0wily
Candidate: 2:16.0~git20160220.1654-final-0wily
Version table:
*** 2:16.0~git20160220.1654-final-0wily 0
500 http://ppa.launchpad.net/team-xbmc/ppa/ubuntu/ wily/main amd64 Packages
100 /var/lib/dpkg/status
15.1+dfsg1-3 0
500 http://se.archive.ubuntu.com/ubuntu/ wily/universe amd64 Packages
holm@frontend:~$ sudo apt-get install kodi="15.1+dfsg1-3 0"
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '15.1+dfsg1-3 0' for 'kodi' was not found
holm@frontend:~$ uname -a
Linux frontend 4.2.0-30-generic #36-Ubuntu SMP Fri Feb 26 00:58:07 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
holm@frontend:~$