I've seen that happen when the humidity is above 90% or so.....OR if water gets spilled or sloshed onto the little sensor on the unit.
What kind of substrate do you have? If you're using the shredded coconut husk or other damp substrate and set the unit right on top of it, it will also read "hi" for the reasons just stated.
Make sure no water has spilled on it....and let the enclosure air out a bit...see if it gives you a number reading in a little while.
I would pull it out an see what readings you get in the room. The only one I have had do that for any length of time got water spilled on it, and eventually it got back to normal and seems to be working fine now. Your enclosure may just be really humid, but if it says the same thing out in the open air, I would take it back for another one.