Some addendum. While digging in the system log files I found many errors like these ones:
2018-06-15 11:45:30.678 [ ERROR] linuxdvb: Availink avl6862 #0 : DVB-T #0 - failed to config dmx for pid 1151 [e=Operation not permitted]
Jun 15 11:52:18 CoreELEC tvheadend[2939]: linuxdvb: Availink avl6862 #0 : DVB-T #0 - failed to config dmx for pid 850 [e=Operation not permitted]
Jun 15 11:52:18 CoreELEC kernel: DMX: too many channels
May be related to the problems I found?