The sensor malfunctioning would be my guess. I notice the temp is unaffected which suggests to me your humidity sensor may be going. The console does nothing but report the humidity. It does not calculate it whatsoever. So I wouldn't put that in the mix here.
I'd start with a new sensor which is the cheapest of the 3 (sensor, ISS, console) to replace at about $50 (using galfert's link above).
If it continues I'd suspect the ISS. A new SIM board for the ISS will run you about twice that.
