The Pro is meant to be used indoors during home inspections in rooms where occupants would spend most of their time.
If you used your device in humid environments and are getting error messages, remove the Pro and set the device in a dry environment for 24 hours before trying a reset.
When doing an official radon measurement, remember to stay within the Operation environment:
5% Relative Humidity to 85% RH non-condensing
39°F to 104°F / 4°C to 40°C