Chances are you may have the type of temp/humid sensor with one of these inside since you said that humidity only went bad
. It may have a plastic grill/case enclosure snapped on over it. These are what read the humidity from a special hygroscopic gel/compound applied on it's face. This gel over time can lose it's properties or change causing the humidity readings to go bad. I have been unsuccessful locating a source for replacement gel but did discover certain Polyethylene Glycol gels have the right properties. Polyethylene Glycol is found in many hair gels and it's properties vary as indicated in the listed ingredients, such as PEG-8 or PEG-45, etc. It appears the number has some relation with the moisture absorption and evaporation rate. I have found gels with around PEG-8 to 12 (or lower) seem to work best. I've read a study that states the higher the number the lower the hygroscopic property.
http://www.arpc.ir/Portals/0/Pdf/Products/Chemical/PEGChemicalGrade.pdf So if you do find you have these resistive humidity sensor chips inside, you can clean off the old gel carefully with a q-tip soaked in alcohol and apply thin layers of hair gel (with PEG), letting it dry between applications, while watching the console/display readings until you're back in business.
There are also the Sensirion SHTxx type of sensors
used in many units today and measure both temp and humidity together on the chip. Since you mention only humidity readings are suffering you may have the first type I mentioned above. Good luck and post results if you do go for it.