I had the same problem with my thermostat. It worked for a while and now it dosn´t. I have changed the line:
String WohnzimmerHeatingCoolingMode "Modus" (gWohnzimmerThermostat) [ "homekit:HeatingCoolingMode" ] { channel="avmfritz:FRITZ_DECT_300:192_168_123_254:119590367200:radiator_mode" }
to
String WohnzimmerHeatingCoolingMode "Modus" (gWohnzimmerThermostat) [ "homekit:HeatingCoolingMode **" ] { channel="avmfritz:FRITZ_DECT_300:192_168_123_254:119590367200:radiator_mode" }
and now it worked again.