Tvheadend

Hello forum members,

I'm using tvheadend since years, the recent version since weeks, with no problems so far. But since yesterday noon, it immediately stops working. Nobody changed the config in that time frame, because nobody have been at home. Went back from work, my wife found "monit" (monitoring tool) flooded our mail inbox since hours, telling "tvheadend start action failed".
So I did it manually:

$ sudo service tvheadend restart

$ sudo service tvheadend status
● tvheadend.service
   Loaded: loaded (/etc/init.d/tvheadend; bad; vendor preset: enabled)
   Active: active (exited) since Sa 2018-05-05 07:46:14 CEST; 2min 16s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 11133 ExecStop=/etc/init.d/tvheadend stop (code=exited, status=0/SUCCESS)
  Process: 11152 ExecStart=/etc/init.d/tvheadend start (code=exited, status=0/SUCCESS)

Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 12012V in 19.2 Astra (0x559635f133e0) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 12441V in 19.2 Astra (0x559635f01f00) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 12129V in 19.2 Astra (0x559635ef0670) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 12581V in 19.2 Astra (0x559635eecb60) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 11797.5H in 19.2 Astra (0x559635ee8710) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 11914.5H in 19.2 Astra (0x559635ee0360) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: mpegts: 10891.25H in 19.2 Astra (0x5596358382e0) - deleting
Mai 05 07:47:21 FileServer tvheadend[11173]: epgdb: save start
Mai 05 07:47:21 FileServer tvheadend[11173]: epgdb: stored (size 138)
Mai 05 07:47:22 FileServer tvheadend[11173]: STOP: Exiting HTS Tvheadend

I can (and did) repeat it a lot of times - no change

so I reduced the number of DVB-S tuners from 4 to 1, changed USB port - no change

2TB of 4TB disk is free
made an fchk, all fine

restored /home/hts from backup 30 days before - same problem

Hardware is

[  444.087912] usb 3-6.4: Product: PCTV 461
[  444.087916] usb 3-6.4: Manufacturer: PCTV
[  444.088673] em28xx 3-6.4:1.0: New device PCTV PCTV 461 @ 480 Mbps (2013:0258, interface 0, class 0)
[  444.478318] em28xx 3-6.4:1.0: Identified as PCTV DVB-S2 Stick (461e) (card=92)

Linux FileServer 4.13.0-39-generic #44~16.04.1-Ubuntu SMP Thu Apr 5 16:43:10 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Version is
HTS Tvheadend 4.2.5-43~ga2fab13
then updated to
HTS Tvheadend 4.2.6-7~g5613551

same problem, always I get:

Mai 05 07:47:21 FileServer tvheadend[11173]: epgdb: save start
Mai 05 07:47:21 FileServer tvheadend[11173]: epgdb: stored (size 138)
Mai 05 07:47:22 FileServer tvheadend[11173]: STOP: Exiting HTS Tvheadend

Could anyone lead me to next analytic steps, please?

Thank you very much in advance!
Laser-Man