All was OK, and overnight my DS18B20+ESP8266 online thermometer recorded the 10'000th uninterrupted sample.
But this morning at 11h35 after 10'435 cycles the counter variable was reset. Another reset occurred at 13h50. Either AC power, WiFi or xDSL line were disconnected, or several of them together.
The firmware needs new features that help keeping track remotely of failure reasons. Back to coding, then!