BME280 gets stuck to 100% Relative Humidity

We have about 100 dustsensores with a Bosch BME280 as RH/BP and T sensor for sensing  ambient conditions. After a while (some months) and a long period of high humidity, the humidity value gets stuck to 100% for some time. After a while the RH drops back to "normal". Still after a while and after beeing stuck on 100% for serveral times the RH signal drops to 0% and no response from the RH signal anymore. What could this be. We operate within the conditions written in the specs.What could this be??