Since the delivered example of the RTC library is working, there might be somewhere a memory overflow in the sketch which gets "visible" due to some different compiler behavior.
Found this in the changelog:
ARDUINO 1.8.6 2018.08.23
[ide]
* Updated arduino-builder to 1.4.0:
- parallel build (improve compile speed)
- better reuse of already compiled files
- Windows: fixed build for sketch stored under OneDrive cloud folders
Found this in the changelog:
ARDUINO 1.8.6 2018.08.23
[ide]
* Updated arduino-builder to 1.4.0:
- parallel build (improve compile speed)
- better reuse of already compiled files
- Windows: fixed build for sketch stored under OneDrive cloud folders

