your supercap on your ISS board out doors could be failing. there are two theories, one is to replace the supercap if you are handy with a solder iron and the other is to replace the unit.
In the US, there are a number of places that sell it cheaper than Davis, my favorite is Scaled Instruments in Florida. Good price and great service. The new board comes as the back half, with a new antenna. You have to pull out the foam plug and feed your sensor wires out. Hang on to the solar panel as it doesn't come with one. Davisnet has a video on how to replace it. I have done it twice and it is no big deal. The fact that the video exists seems to tell me that this is a thing that happens. I see you are in Portugal and there are places that someone here might suggest to go for parts, or to order it internationally. Make sure that if you do buy the ISS , it is of the same type as your receiver, that is if it is EU, then you have to get the EU version
I guess there is a third option, you can bat this around with various retired engineers that frequent this board for several weeks or month, before you decide to either replace the super cap or the board. I would replace the board as it is possible that the transmitter is also failing.
I did replace the supercap myself. I had broken my arm in a winter fall, so I bought a new ISS unit and then when all was healed, put in the new supercap and now use it for a soil surface thermometer and it works fine.
Your choice , and you are welcome.
