I let it run a bit longer, and apparently the TVHeadend errors are synchronized with "Poor Quality" lines in dvb-de-tool:
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 30.98dB UCB= 56158707 postBER= 674x10^-9 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.03dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.15dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.13dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.19dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.07dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 30.95dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -38.00dBm C/N= 31.03dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.03dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.18dB UCB= 56158707 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.13dB UCB= 56158707 postBER= 0 PER= 0
%{color:red}@Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 31.36dB UCB= 56158751 postBER= 93.1x10^-6 PER= 2.20x10^-3@%
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.14dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.27dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.17dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.27dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.02dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.38dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.17dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -38.00dBm C/N= 30.96dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.25dB UCB= 56158773 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.06dB UCB= 56158773 postBER= 0 PER= 0
and after a long time:
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.02dB UCB= 56159233 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 30.92dB UCB= 56159233 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.10dB UCB= 56159233 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 30.92dB UCB= 56159233 postBER= 0 PER= 0
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 31.07dB UCB= 56159233 postBER= 0 PER= 0
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 30.80dB UCB= 56159317 postBER= 110x10^-6 PER= 2.80x10^-3
Lock (0x1f) Quality= Good Signal= -37.00dBm C/N= 30.81dB UCB= 56159317 postBER= 0 PER= 0
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.83dB UCB= 56161117 postBER= 2.42x10^-3 PER= 60.0x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 30.86dB UCB= 56161954 postBER= 318x10^-6 PER= 7.90x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.70dB UCB= 56162677 postBER= 961x10^-6 PER= 24.1x10^-3
Lock (0x1f) Quality= Good Signal= -38.00dBm C/N= 31.01dB UCB= 56162677 postBER= 0 PER= 0
Lock (0x1f) Quality= Poor Signal= -38.00dBm C/N= 29.39dB UCB= 56164210 postBER= 2.07x10^-3 PER= 51.1x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.99dB UCB= 56166370 postBER= 2.87x10^-3 PER= 72.0x10^-3
Viterbi(0x07) Quality= Poor Signal= -37.00dBm C/N= 31.02dB UCB= 56166699 postBER= 666x10^-6 PER= 16.5x10^-3
Got parameters for DVBT:
FREQUENCY = 522000000
MODULATION = QAM/64
BANDWIDTH_HZ = 8000000
INVERSION = OFF
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
GUARD_INTERVAL = 1/8
TRANSMISSION_MODE = 8K
HIERARCHY = NONE
DELIVERY_SYSTEM = DVBT
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.11dB UCB= 56169384 postBER= 3.31x10^-3 PER= 82.7x10^-3
Lock (0x1f) Quality= Poor Signal= -38.00dBm C/N= 29.28dB UCB= 56171100 postBER= 2.30x10^-3 PER= 57.2x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.26dB UCB= 56173713 postBER= 2.55x10^-3 PER= 63.4x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 28.99dB UCB= 56175735 postBER= 2.72x10^-3 PER= 67.4x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.11dB UCB= 56177631 postBER= 2.53x10^-3 PER= 63.2x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.27dB UCB= 56180366 postBER= 2.76x10^-3 PER= 68.9x10^-3
Viterbi(0x07) Quality= Poor Signal= -37.00dBm C/N= 29.32dB UCB= 56181678 postBER= 2.63x10^-3 PER= 65.6x10^-3
Got parameters for DVBT:
FREQUENCY = 522000000
MODULATION = QAM/64
BANDWIDTH_HZ = 8000000
INVERSION = OFF
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
GUARD_INTERVAL = 1/8
TRANSMISSION_MODE = 8K
HIERARCHY = NONE
DELIVERY_SYSTEM = DVBT
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.55dB UCB= 56183487 postBER= 2.42x10^-3 PER= 60.3x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.20dB UCB= 56186533 postBER= 2.98x10^-3 PER= 74.6x10^-3
Lock (0x1f) Quality= Poor Signal= -38.00dBm C/N= 29.41dB UCB= 56188246 postBER= 2.30x10^-3 PER= 57.1x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.16dB UCB= 56190289 postBER= 2.74x10^-3 PER= 68.1x10^-3
Lock (0x1f) Quality= Poor Signal= -37.00dBm C/N= 29.16dB UCB= 56193137 postBER= 2.97x10^-3 PER= 74.3x10^-3