Unit is still working, after so many years of operation in wind, snow, haze, and scorching sun.

What failed first is humidity sensor, that was already showing signs of age with it's weird readings as it turned out after all. Here is a picture of the old one:

Datasheet of such sensor HS1101 specifies minimum capacitance to be around 160pF for 0%. No wonder it was giving such erronous readings before. Outdoor unit is equipped with a module that holds also the calibration data (EEPROM with mcu), but fortunately for my unprofessional outdoor humidity measurement, changing just the component gave good enough reading. New one:

After replacement outdoor unit gave a reasonable readings again.