Tvheadend

I have an issue that appeared twice already and last time I was only able to recover it by reinstalling the server.

Everything worked until now since half a year, with no updates whatsoever on TVH server.
From one day to another some of the clients (4 out of 9) stopped working, they seem to connect, but disconnect immediately.

This is what I see in TVH log:

2016-06-29 16:34:34.424 htsp: Got connection from 10.11.12.10
2016-06-29 16:34:34.467 htsp: 10.11.12.10: Welcomed client software: Kodi Media Center (HTSPv25)
2016-06-29 16:34:34.582 htsp: 10.11.12.10 [ Kodi Media Center ]: Identified as user 'lcseh'
2016-06-29 16:34:34.582 htsp: 10.11.12.10 [ lcseh | Kodi Media Center ]: Privileges updated
2016-06-29 16:34:35.073 htsp: 10.11.12.10 [ lcseh | Kodi Media Center ]: Disconnected

On the other clients it works fine, and changging to a new user doesn't help...

Has anybody seen such weird behaviour? I would appreciate any suggestion, that would help avoiding a reinstallation...
Or at least some points where to start debugging in order to find the root cause?