I managed to get the Arduino UI running with the CORE2 (using the M5Unified 0.1.9 and the Sensirion I2C SCD4X 0.4.4 libraries).
I took the example ino file and modified the header to use M5Unified.h instead of M5stack.h... and the problem seems to be there as well.
The first call to
scd4x.stopPeriodicMeasurement() returns an error saying:
Error writing to I2C bus. Then it hangs apparently on the subsequent
scd4x.startPeriodicMeasurement() call, for what I can say.