Heads-up: I've observed my sensor trying to sleep for 4 billion ms when running the sample code:
That's 2^32-82. So I think there's an underflow with this line:
unsigned long sleeptime = BME280measurementSleepTime - (quicktimecheck - previousBME280Millis);
Not sure where exactly though. I've changed my code to simply sleep for a constant 60s.