After the humidity sensor, next to fail was a solar panel, which under the influence of elements started showing signs of wear. A close inspection shows cracks on the plastic cover protective film.

No wonder, it completely lost its ability to convert sunlight into electricity.

Nominally it should output around 6V with no load, and around 100mA under load, but was not able to produce any current when loaded with 40 Ω resistor.

At the same time, the rechargeable battery seems to survive OK, but since I already have it on my desk it will be replaced as well.