Before I get to much further I know others will be looking for help on the same Issue. Before using my solution please work through this tutorial first to eliminate the obvious stuff first.
If you find you have correct temperature but you still have the 99.9% humidity please read on!!
I did some research on how the am2302 sensor sensor is constructed and how the sensor functionally operates.
There is a moisture holding membrane between two electrodes. As per the URL provided.
Should this layer become saturated you will get a readout of maximum resistivity, Hence the 99.90% read out.
So the solution is to "dry" the sensor out.
An example of how I did this was by submerging the sensor methylated spirits for a couple of hours and then left to dry ( dry heat works best, do not blow dry other wise you will end up right back where you started form the moisture in the air).
I also tried putting the sensor in an over night in a container with a moisture absorber (damprid) which also worked for a larger group of sensors.
If this solution worked for you please ADD some karma TIA 