I can't reproduce this. Have you tried deleting that rule and creating from scratch? If so, maybe it's something with the device. You could try creating a virtual thermostat and see if you still get the error using it.
What does "Current States" look like on the devce detail page for the affected thermostats? My guess is that the possible operating and fan states are not in the new JSON format (which is supposed to worked either way for the moment, but it's possible something was missed). On a related note, what drivers are they using?
Thanks for the suggestions. The example was with a new rule. With a virtual thermostat as you suggested it worked. With either the Heatmiser Neo Thermostat or the Unified Thermostat it fails with the error. Heatmiser Neo is the one I really need to get working
Is this a custom driver? It's missing the supportedThermostatFanModes and supportedThermostatModes attributes, which I'm guessing is tripping up the rule since it's probably trying to read them to know what to present in the UI.