My DataMate is giving strange readings. The humidity in the box was 90%. Could this be the problem?

Yes. Humidity greater than 60% can cause problems, particularly when the DataMate undergoes temperature changes, such as being taken from a warm office to cold outdoors or from hot outdoors to air-conditioned office. If possible, store the DataMate in a cool, dry place in winter and in a warm, dry place in summer.