Heads-up: I've observed my sensor trying to sleep for 4 billion ms when running the sample code:

60082 MCO:SLP:MS=4294967214,SMS=0,I1=255,M1=255,I2=255,M2=255

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.