Just flashed the 5.1 firmware without any problems. The LEDs now work again and return water temperature is being reported. Also updated the otmonitor on my RPi to version 5.1.
When I use that software it shows that 'Flame status' is ON, 'CH mode' is ON and 'CH enable' is ON and 'DHW mode / enable' are OFF. Just checked on the boiler itself but it is not burning.
So this seems not to be wright. OpenTherm version slave is reported as '3.00'. The status field reports: 00000001 00001010
Another strange thing is that when I go to 'Options' -> 'I/O pins' the GPIO for port A and B report 'none', the 'LEDs' are reported correctly. But the returnwater temperature is being reported so it seems to work.
When I set GPIO_A to Vcc and GPIO B to Temp sensor and press Done all seems to be ok. When I then open the I/O Pins configuration again the values are correctly shown. This also was the case wuth the 5.0 version.
Thanks for the updated 5.1 version!