We got our RPi 5 today and found the issue. Basically RPi 5 has changed the gpio chip name. So this line:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/gpio.cpp#L475
the name “gpiochip0” should be changed to “gpiochip4” on RPi 5. We will be working to address this and make the firmware compatible with all RPi versions. In the meantime, you can modify the name yourself and recompile the firmware, and see if this fixes the issue for you.